Why does this guy do this?

This guy in my newest class is always starring directly at me. Like so obviously! I look up he's starring I loon right at him and he's just smiling at me starting so I usually smile and stop looking at him. Recentely I found out hey he's not single...I didn't know because his gf is at a different school. Oh and once I walked into his class to talk with a teacher the second I walk in he doesn't stop looking at me he's turned around in his chair just starring. Haha and once we leave my friend is like...dude who was that guy starring at you? Why does he do this?