I think the answer to the 'why' is simply - you just are. It's the way you are, in this respect. Now, how do you let go of the thoughts, the jealousy? Yes, that's the tricky part. I use mindfulness on a daily basis to help let go of troubling thoughts within the moment. May not mean you can let it go for good, but when the thoughts come back, you just use the mindfulness again. It's a way of lessening the impact of obsessing, I guess you could say. Mindfulness. Maybe you've heard of it.