Why would a guy who's friendly otherwise, act all aloof around other people?

Theres this boy who used to like my friend and follow her around all the time. She blew him off and hooked up with someone else, and he and I became quite friendly meanwhile. Now hes very friendly over IM and keeps telling me to treat him, and hang with him. However, when I see him in college, he's very friendly sometimes, but acts very weird and not friendly sometimes, especially I think, when there are lots of other people around. Why is he acting so strange?