Why does the same boy keep coming back to me, and then a few days later just treats me like crap all over again?

Maybe he is seeking attention but no matter what his intent is he has no right to treat you in such a manner if he is fickle with his intent and in the end, rude or abuse he doesn't deserve you and you deserve better. He might be working through issues of his own but until he gets to an emotionally mature point and is able to treat you the way you deserve it is not your job to put up with him in any way shape or form. No matter the reason it is not a situation you want to be in.

Well based on some experiences, maybe he just needed someone to vent about some things or to do things without any complaint and you can give those comfortable feelings to him, but after he has done whatever he wanted, he just doesn't really care about anything anymore and treats you bad again and again. He keeps doing that because he knew you, he knew you would let him back to you. So, I think it's your choice if you want to keep him coming back to you or not. No one deserves to be treated like crap. I'm sorry if I misunderstood about things that happened but it's just what I'm thinking.