enfin, yes I think it is common to obsess over someone who has entered our emotional orbit. I have to guard against obsessing over people, too.
It's all part of the spinning brain of obsessive people, in my opinion. Like C23 said, mindful thought is possible. I have to break obsessive thinking in a lot of areas - food, TV, SoberRecovery, people who have entered my orbit, worry, fear, you get the picture.
If it's that guy...that is really self-destructive. Maybe a little therapy would help. I found it was particularly bad in early sobriety. At about a year, my thoughts became much less anxious and more manageable. It takes some inner work and careful thinking.