Why does he act like this towards me?

My Ex boyfriend said to me last yr when we broke up he wanted to be strangers and that was it. I still don't know why we broke up though. Anyways this year we have the same math cl and homeroom. We are both seniors now. The first day I walk in cl and say hi to the teacher he smiled my way then sat back down. One day in math cl the teacher messing around touched my hair and he doesn't like the teacher that much and told him when we were dating not to touch me. Well when he touched my hair he was in the back of the room saying dude that's not right you shouldn't touch her hair and ranting on to the girl in front of him. He looks at me sometimes like glares. When our friend in cl mentions about we should date again he says no or hell no. When I open the door for him he just looked at me and walked behind me. I got his paper for him out the printer one time and he just took it from me without saying anything to me I don't get him. Why does he act like this? I never do anything bad to him I just act normal.