Why do I miss my exes?

I have a girlfriend now that I love immensely, however I find that sometimes when I stop and think, especially when listening to music that reminds me of those times or even unrelated love songs, I find myself picturing each and every one of the girls I was emotionally connected to, even if the relationship was one week long. Then it turns into a feeling of loss, and a need to see them, anything to get in touch. Just wondering if there's a neurological explanation for my "re-infatuation." And if there's a way to suppress or even eradicate these uncomfortable feelings.