Why am I attracted to guys but only when they are taken?

It's not like that I notice them only when I learn they have a girlfriend, just so we are clear. I can see them in school, out of school, wherever and not give them a second glance. But when I see them with some girl I'm like "daaaamn" BUT i never make a move. I just get a kind of a crush on them... :/ The last half a year I'm crushing on a guy since I heard him talking about a girl (just his friend atm) that he liked. They're in a relationship the last month or so... I know it's weird. A lot may think I have low self esteem or something but it's nothing like that. I don't even know! I try to be subtle. Not looking or flirting at him. But why do I do that in the first place? Please, any help?