No matter how good she looks, some other guy is sick and tired of putting up with her.
I saw that on a bumper sticker...
A friend of mine says it this way: "Show me a hot chick and I'll show you a guy who's tired of f*****g her."
As told to me years back:
My buddy married the hottest girl from school. She had remained very attractive. Buddy 2 said "I'll bet you bang her EVERY night." Buddy 1 said that he probably yanks thecrank just as much as any other married guy.
Moral of the story - I don't know, I'm just relating a story I heard.