Can never be alone?

What is it narcs that they can never be truly single? My ex went from his wife (who he cheated on) to living with a woman (who he cheated on) to me (well, you see the pattern) and is now in another serious relationship. I'm content to play the field and have fun and not settle down with the first person who happens to be available but he's isn't.

I hear this is common among narcissists and wonder why that is?