Why do people feel an urge to confess to affairs?

In this question, we were asked if we would tell a friend that their SO was having an affair. Most people say they would tell.

My question is why someone who was having an affair would tell anyone—much less a close friend who was also friends with the person they were cheating on—that they were having an affair. Why would you put your friend in a position of either having to keep a secret or of divulging the secret?

Are people unaware they are putting their friendship at risk? Do they secretly want to have their SO told what is really going on? Do they not realize the position they are putting their friend in? Do they think their friend will happily keep a secret? What is going on?