Why can't i like him?

Okay so there's this guy I really dont like, like deep down I can say I hate him. Our personalities dont match at all and we have gotten into it a few times before. Time has passed and I've tried to forgive and forget, but I just can't seem to get along...

with him. And I know that if I dont like him I should just try to avoid him but he's like my boyfriends best friend. They spend more time together than we do they have more in common than we do and I guess some of it is jealousy. My boyfriend knows how I feel about him but he thinks its me being close minded and holding a grude ok so if I am. I mean just knowing they are going to hang out pisses me off makes me want to cry. my personality just wont mash with his he carries himself as if he's better than me better than half the world. He talks down to everyone including my boyfriend. im afraid he's going to get to the point where he feels comfortable enough to talk about me to my boyfriend and he (my boyfriend) wont defend me. it's building a gap between me and my boyfriend and I love him more than I've ever loved anyone and I feel his friend is going to cause us to split

You aren't expected to like everyone - personality differences can most definitely make you not like someone. I know what its like to not like someone, but you can put up with him if you try. Just make an effort, doesn't mean you have to be friends with him. When hes around have a neutral conversation with him, don't be a snob and just not say Hi or anything. When your annoyed by the things he says/does just hold it back, you'll get over it.

im not a snob to him im very nice to him but my boyfriend can see im not happy then he'll ask about it and we start fussing and get mad and he leaves to go chill with him

Well at least you are trying, does your boyfriend not realize that?
Just tell him its something you are trying for and you need to get used to the problem. It sounds like the root of this problem is your boyfriend because he thinks what you're doing isn't good enough.Just say If you want me to like him, you're going to need be more patient with me or else its just going to get worse.
Hope things work out.