For me, a sign of maturity is learning to recognize that disliking someone so strongly in no way improves my life. Harbouring a grudge and fervently clinging to hatred only makes me a negative person. Disliking them does not make me happier, richer, more in love, more at peace, funnier, more successful or more awesome. If anything, it detracts from my ability to find and be all these things.
Life happens - you meet people you like and you meet people you don't, relationships grow and they fall apart, friendships struggle and thrive, things happen to make you happy and also to make you cry. This is life. You're not expected to love everyone and everything, but I think we're expected to approach life with an open mind. And we're not expected to forget everything bad that happens, but I think we owe it to ourselves to move past it.
Being angry is not a 'right' that I think I deserve because of the way the cards have been dealt. I sometimes think it makes me feel better, but this peace feels much better. Letting go of the things that I can't change brings a sort of relief I didn't even realize I needed.
Let's save the anger for more important things - you know, like people who don't signal their lane changes, burnt cookies or when the DVD skips.