An Affair?

What is it about certain people that they can't stop having affairs?
One of my cousins is like that.  She's had a hand in breaking up more marriages then I even want to think about. Her own included.

Why?
Was it the thrill of sneaking around? Was it the challenge of taking someone from someone else?  Or was it just loneliness in her life?

We see it in books, movies and tv shows all the time.  The lead character falling into an affair usually with the reason of a better chance at romance, and what is it about us as a society that we accept affairs so openly?

In the third season of SATC, Carrie has an affair with Mr. Big and the only one of her friends that tells her to think about what she's doing is Charlotte. The rest of them accept it like she just told them their out of toilet paper.

I hear more often then I care to, about the people around me having affairs. And I can't help but wonder what it is in their lives that would lead to an affair? 

In the end are we all just relationship intolerant or is there something deeper going on?