What's the psychological explanation of a "Homewrecker"?

It is beyond my comprehension when others completely disregard relationship statuses. I would never yearn for a man who was already in a relationship. I don't understand how some women/men actively pursue a person knowing they're taken. Can anyone explain what the possible reasons could be for another wanting to ruin a relationship? What is the mindset like of this homewrecker? How did they turn out to be like this?