Why can't I stay angry at anyone?

There's nothing wrong with you, it's just your inherent personality type. You're obviously very open-minded and understanding, and also don't like conflict and like to have any conflicts quickly. This can sometimes be damaging, if you tend to bend over for someone who has wronged you and puts you in a "doormat" position. Unless something is seriously affecting your life, something that's causing you extreme anxiety or causing issues in your relationships, there's no real need to see a therapist, unless you simply WANT to see a therapist, which is totally fine.