I make it a habit of not keeping contact with exes. We go our separate ways and that's it. I check in on their FBs once in a while to see if they're happy and alive but that's it. Only one ex do I keep in contact with. This girl was my first sexually, and we were together for only a few months in 2006. It ended badly.  She got pregnant, never told me until months after we split, had an abortion. She's the only woman to bear a child of mine to date even if briefly. I've had more real relationships since. Been engaged. Met more intelligent women. Prettier ones. I've been with my girlfriend two years now but every time I look at my ex or talk to her I wonder what if. What could've been. And I kinda wish it could be. It doesn't make sense, even to me, after so long. We're probably very different. But the memory of that short, wild, turbulent time makes me happy, and I just kind of wish we could get coffee and pick up from there at times.

Why do I feel this way? Is it normal?

I feel that way sometimes about a person I briefly dated in 2015 and had a bad breakup with  :-\\

I'm sure it's "normal", and if you don't act on it, it can't be harmful either.

Of course its normal, especially in  the case you describe.  I occasionally think "what if" about girls I dated in high school