"Let the lie come into the world, even dominate the world, but not through me"

— Alexander Solzhenitsyn.

Here's why it's a bad idea to lie to either your friends or your enemies.

Firstly, you shouldn't lie to your friends:

• Lying to your friends harms them inherently. Your friends assign inherent value to the accuracy of their beliefs. It's not friendly to mess with them like that.
• Lying to your friends also harms them instrumentally. When your friends have inaccurate beliefs, then they are less likely to achieve their goals.
• And there's a third consideration. Lying to your friends disempowers them. That is, you diminish their ability to reliably influence their environment. It's as if you had stolen their car keys or locked them in a cupboard.
  
  Not only must you care that your friends achieve what happens to be their goals, but you must also care that your friends are situated to achieve their goals whatever their goals might be. For example, if you steal money from a bank and give the money to your friend, but you tell your friend that you won the money in a poker game, then even if you have increased the likelihood that your friend's goals will be achieved, nonetheless you have placed your friend in a position such that were their goals different then they would not achieve those counterfactual goals.
  
  This Empowerment Condition is a necessary condition for a relationship to be "friendly". (There is a difference between alignment and friendship.)

Okay fine, don't lie to your friends. But what about everyone else?

Don't lie to them either.

Lying is a poor strategy even on Machiavellian grounds.

• You might think "If someone isn't my friend then what do I care whether I disempower them? If they're a stranger then I'm indifferent to their disempowerment, and if they're an enemy then I want to disempower them!"
• The problem is that people are likely to know things that demonstrate that you are lying. So you might tell someone $X$ despite knowing that $X$ is false, because you hope that they'll believe $X$ based on your testimony. But actually, they know $Y$ which is mutually inconsistent with $X$. So now they know $\neg X\wedge Y\wedge \text{You Are Dishonest About}\left(X\right)$.
• Often this last fact is more important information than whether $X$ is true. The most important fact about a lie is not the lie itself, but rather that you lied.
• Moreover, if the lie is public (or can be made public), then you will get a reputation for dishonesty and no one will trust your testimony.
• And not even your friends will trust your testimony. You might hope to explain to your friends "no, don't worry! I only lie to my enemies. I am honest with my friends." But how can you convince your friends that they are your friends? Telling them "you are my friend" won't convince them because that is also what you would say if they weren't your friend. You've Löb's theoremed yourself in the foot.
• If you are known to lie in a situation $X$, then no one will trust any of your testimony, even in situations $\neg X$, because you will not be able to convince anyone that the situation isn't $X$.
• When no one trusts your testimony, you lose the ability to be either honest or dishonest. You lose the ability to assert anything whatsoever. Your utterances because complete gibberish. You are merely mumbling syllables.
• If you lied to your friend yesterday about something small, causing them to believe it, then their belief was unjustified. (At least, this is the standard view in the epistemology of testimony.) But it's worse than that — all their beliefs based on your testimony become unjustified. So if tomorrow you want your friend to know you love them, then you can't. You can say "I love you", and cause them to believe you love them, but their belief won't be knowledge, because the belief relies on the false premise that you are honest. The unwarrantedness of yesterday will leak into today.
• And most egregiously, all these tragedies don't just befall you. These tragedies befall everyone who is thought to be like you.

Fortunately, while honesty is the most important virtue, it is also one of the simplest to follow. Starting now, don't say things that you know to be false.