Please do yourself a favor and get out of there and that relationship as soon as possible. If he is hitting you he surely does not love you and it will get worse. The longer you wait the more abuse you will face. You do not deserve that.
To try to answer your question of why is he doing this to you, it's evident he has some deep personal anger issues and is taking them out on you. Whatever his reasons are doesn't matter, he apparently is incapable of having a loving relationship with anyone.