You’ll resent friends less
When we don’t speak our minds, we wind up harboring resentment. If you have a friend who you feel talks about herself too much, ditches you for men when she gets drunk, or is selfish about favors (always asking for them but never returning them), say something so she has a chance to remedy it. And maybe she won’t, and you can let her go as a friend. Either way, you release the resentment when you speak up.