How do I avoid bitterness?

It seems like every single person I have ever been close to in my life, I have become angry at them, no matter how mature I think I have become, or how much I like the person at first. It is an anger that comes and goes, but how can I prevent it? Why does it happen? Does it almost always root from selfishness, or just a mismatch of personalities? Is it just me, or is this something that everyone deals with inevitably? Is it something that you accept and move on? Or do i have a bad temper? It seems like the more mature a person is, the less personal they take your bitterness, and less bitter they are too...how do I become like this?