This boy makes me hurt?!?

Well if you think about it 15 and 12 are not that far apart. But even so, thats a pretty big difference at a young age. He has been around a little longer and like you said has done things he might not be proud of. That might be why he reacted the way he did. My advise is that you should just be friends with him because you are still young. it might help to just confront him and say you are sorry for whatever you did (even if you think you have done nothing wrong) and you don't want anything to be awkward between you two and would like to be friends. Hope this helps:)