Why can't I get along with my little brother?

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OK, My little brother is 15 years old and we used to be really close. The past few years he has started treating me like junk(literaly). It seems like he doesn't care about me and he is always insulting me. If I say something like "ewww" or "gross" he says quite looking in the mirror. I try so hard to get on his good side but he just insults me even more. we are only a few years apart.