I read the article "All Memory Is Not Created Equal—Positive Memory Seepage" which talked about how some memories are more easily accessed and remembered than others. The article talked about intrusive thoughts and post-traumatic stress and how it is not only negative thoughts that can be intrusive, but positive thoughts as well.
Positive memories are the ones that people go back to, because they are associated with something good and people have a more stronger attachment to these thoughts because of this. The bad or negative memories however are stored in a different part of the brain that is not so easily accessed. The article talked about this and the role it has in relationships, especially for women. It is harder to remember all the bad parts about a relationship.
The article also talked about the term, cognitive distance which is when you can't hold both the bad thoughts and the good thoughts at the same time and can actually take quite a toll on the persons general health, making it harder to eat and sleep and function. I found this article to be very interesting in that it showed the more psychological aftermath of breakups in relationships and better explained why people react and act the way they do when it comes to relationships.