For those who have been in that situation: meet new girl, couple dates, everything is going great, you can see it leading to the next stage but then he comes back. She falls for his "I've changed' scheme and goes back to him.

Here's why:

"A lot of women (and men) enter into relationships with someone new, before they end their current relationships. It is selfish and unfair to the party who is clueless that she is still in a relationship with someone else! They may say they are split, but if they have not cut all ties and still answering to them the relationship is NOT over. The emotional connection must end as well before they can move on.

I think sometimes they start a relationship with the "upgrade" because they are getting the attention they did not have with the "ex", and it is attention they know they deserve. But at some point, they get scared and think the upgrade may be too good for them, so they run back to what they know. Even if the ex is the worst thing for them. Vicious cycle."