It started with a sense of injustice, that so many people were suffering so I went into international development and was shocked by how expensive and ineffective it was. I burnt-out really badly, and I don't now get much in the way of emotional reaction to many forms of suffering (unless it's personalised and even then it's quite dulled in the moment). I'd seen so much suffering, I knew it was really, really bad, and I wanted to find effective ways to continue to prevent that. When I knew there were routes to actually do this, it seemed impossible not to prioritise that from a moral perspective or an emotional one. Hope that helps!

For me there is a strong "what the world could be if I did this, so it would be a huge waste if I didn't do this" sense that motivates me, although in the past I used to overestimate the potential good effects of my actions. I think it is probably similar to the need for efficiency you mention, but it also generates an unpleasant but correct sense of urgency, because usually if I don't do things fast, the effect could not be the same. There's also wanting to have a good impact in the world, which is more core and generates meaning.

I describe it as a calling. It's not so much that I feel a strong emotion as I feel like it's the most natural thing in the world that I would want to help people and do that in the most effective way possible. Since I focus specifically on x-risk from AI, I find this as a calling to address AI safety due to the natural way this feels like an obvious problem in desperate need of a solution.

For me it's very similar to the kind of "calling" people talk about in religious contexts, and now that I'm Buddhists I conceptualized what happened when I was 18 that made me care about and start pursuing AI safety as the awakening of bodhicitta because although I already wanted to become enlightened at that time (even though I didn't really appreciate what that meant) it wasn't until I cared about saving humanity from AI that I developed the compassion and desire that drove me to bodhicitta. With time that calling has broadened even though I mainly focus on AI safety.

The way I feel when the concept of a person in the abstract is invoked feels like a fainter version of the love I would feel towards a partner, a parent, a sibling, a child, a close friend, and towards myself. The feeling drives me to act in the direction of making them happy, growing their capabilities, furthering their ambitions, fulfilling their values, and so on. In addition to feeling happy when my loved ones are happy, there is also an element of pride when my loved ones grow or accomplish something, as well as fulfillment when our shared values are achieved. When engaging with the concept of abstract people, I can very easily imagine real people - each with a rich life history, unique ways of thinking, a web of connection, and so on...people who I would love if I were to know them. This motivates me to work hard to provide for their well being and growth, to undergo risks and dangers and sacrifices to protect them from harm, to empower and facilitate them in their undertakings, and to secure a future in which they may flourish - in the same ordinary sense that I imagine many other people do for themselves, their children and families, their tribes and nations, all people, all beings, and so on. I feel a sense of being united with all people as we work together to steer the universe towards our shared purpose.

You've italicized "effectively" as part of the question, but I don't think I feel any real distinction between "wanting to help people" and "wanting to help people effectively" - when I'm doing a task, it seems like doing it effectively is rather straightforwardly better than doing it ineffectively. "Effective altruism" does imply a level of impartiality regarding who benefits which I don't possess (since I care about myself, my friends, my family, and so on more than strangers), but it is otherwise the same. Even if I were I only to help people who I directly knew and personally loved in a non-abstract sense, I would still seek to do so effectively.

Among other things, I feel a sense of pride and accomplishment when I do good, the way I imagine that someone who cares about, say, the size of their house feels when they think about how big their house is.

I'm very happy to see this being discussed, and have enjoyed reading others' answers.

Upon reflection, I seem to have a few different motivations: this was a surprise to me, as I expected to find a single overarching one.

a) Imagining another person's experience, leading to imagining what it is like to experience some particular suffering that I can see they are experiencing. Imagining "what it is like" involves focusing on details of the experience and rejecting generalities (not "I have cancer" but "I am trying to reach down in the shower in the morning but can't and the water is too hot"). Soon my train of thought goes to a more objective or detached place, and I think about how there is no real difference between me and the other person, that except blind circumstance there is no reason they should suffer when I do not.

There is an erasure of self involved. I imagine the core of my consciousness, the experiencing self, inhabiting the other person's body and mind. From this one example I generalize; of course I should treat another person's suffering the same as my own, because in the final analysis there is no difference between me and other people. That's the altruism; desire for effectiveness is secondary and instrumental, not terminal.

b) Zooming out and imagining the whole of the world leads to imagining all the evil in the world. (Where "evil" is a broad term including suffering due to carelessness, due to misaligned incentives, due to lack of coordination, due to accident, etc.) It's overwhelming; there's a sense of perverse wonder. "The works of Moloch are as many and burn as cruelly as the white-hot stars." This leads to a powerful feeling of being "fed-up" with the bad things. The desire for them to stop is like a very strong version of the desire to clean up an untidy room. It's abstract and not connected to any one person's suffering. This tends to be a stronger motivating force than a); if a) is empathy, this is anger.

Eliezer's fiction is particularly good at conjuring this mind-state for me: for example, the "Make it stop" scene in http://yudkowsky.net/other/fiction/the-sword-of-good .

This mind-state seems more inherently connected to effectiveness than a), though effectiveness is still instrumental and not terminal. I want us to be making a strong/effective stand against the various bad things; when we're not doing that, I am frustrated. I am less willing to tolerate "weakness"/ineffectiveness because I conceptualize us as in a struggle with high stakes.

A search for root causes, and putting myself in other peoples shoes led me to EA. I did not accept poverty and death in the world as a given, and asking questions about root causes led me down a path of reading and questioning. It helped that I now try to put myself in others people shoes. Trying to read about what people say in their own words. e.g.

Ants Among Elephants: An Untouchable Family and the Making of Modern India by Sujatha Gidla

Eyes off the Prize by Carol Anderson

Deer Hunting with Jesus: Dispatches from America's Class War by Joe Bageant

I agree with the first example--I want to help people effectively because I want to help myself effectively, and I have a deep conviction that others' pain is exactly as important as my own.

When I was young I felt like "Gosh! When I'm older and have a job, I really should use my power as a globally rich person to help those who are much less well off, because that's obviously morally obligatory and this Peter Singer guy makes sense."

When I read Slate Star Codex's "Everything is Commensurable" I thought "Oh right, I suppose now's the time for that, I have more money than I need, and 10% seems about right."

It felt satisfying to be doing something definitive, to have an ironclad excuse for not freaking out about whatever the issue of the day is. "I'm doing my part, anyway."

Then I learned there was a community, was dazzled by how impressive they all were, overjoyed that they wanted to welcome me, and had a strong emotional reaction to want to be a part of it. It was more excitement about the people than the projects. They felt very much like "my people."

Now I don't feel much of anything about it (maybe a touch of pride or annoyance about losing so much money), but I still give my 10% to AMF monthly, and I don't plan to stop, so I guess the earlier surges of emotions did their job.

A few years ago I had very different priorities, pursuing them was not making me happy, and I guess at some point I correctly realized that I'd be much happier focusing on altruism instead.

• I really crave creative stimulation and I see my engagement with altruism as a creative pursuit. I have many creative interests but I expect my interest in altruism to be more stable over time and more socially reinforced, so it gets priority. (That's my narrative, at least)
• I do experience a sense of disgust / antipathy towards what I see as complacency, or failing to engage the world creatively. This isn't necessarily related to altruism but usually I see disinterest in altruism as symptomatic of the kind of complacency that I loathe.
• I really enjoy maximizing / trying to be good at something, so that's a big part of where focus on effectiveness comes in. Strategizing is the fun part of altruism for me.
• I am very sensitive to the idea of being manipulated or pressed into conformity, and disgusted when I view people as falling victim to this. For this reason I am skeptical of many memes about what altruistic behavior should look like, and prefer a "rational" approach that is harder to manipulate.
• I do experience feelings of injustice with respect to stuff like immigration policy, but I don't prioritize responding to those feelings, maybe because I see them as easy to manipulate or whatever.
• There is virtually no sensation of empathy involved. There is also no sensation of guilt, but there is a sense of frustration when I feel that I am failing to actualize my values.

Edit: After reading some other comments, I'll add that I guess I do feel good about being nice to people close to me, and altruism does generate a similar feeling. I'm hesitant to call this empathy because it's not true that I feel bad about the suffering of distant people, I just feel good about helping.