Pretty sure there's something called imprinting where certain people manage to do that on your mind and you have a hard time forgetting them. It has to be true for ESTPs too I'm. But I wouldn't know. I've always thought that it was just a "me" thing since I'm a drifter who's moved around the world too much .. picking up and losing people / friendships along the way.
For me it's less regret, but more longing to talk to them again.