Why do we compare ourselves?

"She's shiny hair, Style section, Vera Wang, and I'm, you know, the sex column they run next to the ads for Penile implants." {Carrie Bradshaw, Sex and the City, Season 3, Episode 33 "Attack of the Five Foot Ten Woman"}

Why is it that after a break-up, some people compare themselves to their ex's new found significant other? In this particular episode, Carrie noticed an article in the newspaper about Mr. Big's wedding to Natasha. She was upset that Big was now married, after he told her he was not ready to get married, but, the truth is that she was more upset about him being married to Natasha. Why?! Because she was young, beautiful, tall, etc.? But Carrie has so many great qualities, she is amazing, yet she diminishes herself because someone else conquered the man she is in love with.

Why do people torture themselves this way? It's like they are trying to prove themselves to someone else. Or is it that they are trying to prove themselves to themselves?

When you firmly believe that it cannot work out between you and your significant other, why does it still bother you that they are with someone else? Or does it hurt more when someone breaks up with you and then runs off with another? What about when you break-up with someone, knowing you still love them, and then see that they are moving on? Is there still a burning inside? Why? Especially when you are the one that cut it off, why do you compare yourself to the new person in your ex's life? If you did not want to be with them in the first place, why does it matter that they have sound someone new?

In Carrie's case, Mr. Big broke her heart, moved to Paris and immediately got married to someone else. In her mind, she is left wondering "Why?". Why would he tell her he loved her, yet could not be with her and then live the life she wanted to live with him, with another woman? Why? The only thing left for Carrie to do, is compare, wonder, ask...why?...then, she must move on.