Ever think too much about ex girl/boyfriends?

Lately I've been having this weird feeling, almost like nostalgia about the exes in which the relationship didn't end in a burning wreck, and I'm wondering if any of you ever get this... It's happened before and passed, but I don't know why it happens in the first place, I love the person I'm with, but I just get this feeling like I'm left out of something I used to be an important part of and I long to be included, I want to know how they're doing, and it leads to me wondering how things would have been if I would have changed a thing or two. Is this healthy, am I being an *** behind my partner's back?