My best friend is someone I hate being around now.
Why can't you stand them?
It's the way he is these days. He never behaves.
Well, I get you like him, but you shouldn't still hang out with people who make you miserable. If you don't like the person they've become, you should probably move on from them.
I am stuck with him.
You are never stuck with someone unless you choose to be. If you aren't happy with their behavior you should either confront them or avoid them.