Why would a guy try to hide his girlfriend from another girl?

There is a guy I like (I know he likes me too but we don't know each other). The thing is he always tries to hide his girlfriend from me-we live near each other but his girlfriend doesn't and whenever she's round he will totally avoid my house or anywhere where I might be and the other day I saw him and it seemed like he was trying to hide the valentines card he got his gf, I mean why would he do that? It's obvious he's going to get his girlfriend a card so I don't see the point in him trying to hide it from me.

Why would a guy try to hide his girlfriend from another girl?
Post Opinion