I know of two substantially different approaches to deal with the difficulties of the world. The stoic approach is about making oneself accept the things how they are, no matter how horrible, painful and frightening. Acceptance is a rather leveled emotional state, unlike happiness which is a one sided set of emotions. Positive psychology is about creating positive feelings through positive thinking (- instead of realistic thinking?), optimism, hopeful thinking and generally attention control towards positive things.
I think in what sense positive psychology can be useful, is as a toolbox to heal mental weaknesses by balancing a lot of negative emotions with positive ones. However, I don't consider the positive focus and constant happiness as a reasonable or healthy state, but as an intermediate stage, as for some people training to handle and accept all the horrible painful and fearful parts of reality is a very challenging procedure that requires lot of strength. Happiness, I think, is not an appropriate or sustainable state to have in the world that we currently live in. It is a state for moments and parts of life to encourage hard work towards good things. But the state of happiness has the down side of getting addictive and lose the ability to handle unhappy states. In order to not freak out every time the next horrible thing is happening one needs to learn to accept the consistency of change and general lack of control.
You may as well say that stoic thinking could be problematic, since when you accept things how they are you lose drive working towards better things. I do see that risk, and even have several friends who are thinking like that. But it seems to be crucial to accept things how they are to not make one self dependable on the change you are working on for the world. In order to be and stay healthy people shouldn't be driven by the need for change but by the self-fulfillment that comes with investing in one's values.
There may not be much particularly new about my arguments, which is why I am wondering whether my education was/ is just much different to others, or whether I am missing something, that among many effective altruists maximizing happiness is considered a valuable and healthy goal.
I hope people can share their resources for the various arguments, so we can update our knowledge.