I really hated it when my friends would greet my girlfriends by telling them how beautiful they were. They would do this right in my face. At first, I thought nothing of it. As I grew older, I would find that the girlfriend would immediately, if not sooner, jump into bed with that guy as soon as I called it quits with the girl.
Moral of the story: When your friends compliment your girlfriend in front of your face, watch them like a hawk. They’ve already marked themselves.