Why do guys brush up against girls?

I am 16. There is a guy in my cl and my teacher igned me to work on a couple projects in cl together. We both had to stand up to work on the project and he kept standing so close to me when there was plenty of room. He kept brushing his arm against mine very subtly. He did this multiple times. Then, he walked behind me and his shoulder brushed against my back very lightly but there was plenty of room again. He is loud but gets quiet around me and doesn't try to talk to me, but when we do have to talk his voice is a softer tone than when he talks to his friends. What does he want from me? If he liked me wouldn't he try to talk to me? Why does he do this?