Why does he "ignores" my compliments?

So there's this guy I like and he likes me back. he has a girlfriend and I respect that but we are still friends. Once in a while, I'll compliment something about him. Like recently he grew a beard and I told him that I liked it. He then said, "Yeah, my girl wanted to see what I looked like with one." I complimented him on something else a while back and he just says, "Yeah..." and just changes the conversation. It's not as if I'm over-complimenting him or anything. These are two separate occasions. Is this a guy thing or what?