Why do guys in committed relationships still become infatuated with other girls?

I have noticed that a lot of guys, even if they have been dating their girl for a long time and are completely committed to their girlfriends, will still become sort of infatuated with other girls and want to hang out with them. It seems like guys who are in committed relationships will meet pretty girls and become sort of infatuated with them and want to hang out with them, not because they want to cheat on their girlfriend with this new girl, but just so that they can be around a new, pretty girl and have that fresh and flirty thing going on. Why does this happen? Is it natural for guys to want to be with more than one girl, or do I just know a lot of bad boyfriends?