Why does he touch the top of my head?! lol?

It's done in a playful or almost affectionate way but normally I would consider it semi patronizing because he's much bigger than me. Three weeks ago I was sitting in a chair and my guy friend walked behind me and just laid his whole hand on top of my head when he went by then looked back to see if he got a reaction. He's huge BTW, a foot taller than me and his hand covers the whole top of my head. Anyway then next week he came up next to me while I was sitting and put his hand there and left it. That I get because it became a game, I kept trying to swat him off and he kept putting it back until I got up and we sort play fought. Then last week he asked me to help him with something he wasn't understanding in school. When I was done writing on a white board and we were talking he just put his hand on my head for a second again. Why does he do that lol? It's sorta odd but I don't really mind it I guess.