To answer your second question I think it's a matter of wanting what you can't have, and the inverse of that is becoming interested in somebody who is single while you're dating someone else .. Also known as "the grass is always greener" theory I like to call it, but we all know that's not usually the case .. Helps keep your head right when you're in a great relationship
Recently this girl I know was in a relationship with some dude and I couldn't have possibly found her more attractive. Like rocket status. Then she went and broke up with him recently and I'm thinking to myself 'Something's different and not in a good way'.