What makes a person obsess with what another person does with themselves?

I am just curious what gets another person to obsess over another person when it is a negative obsession. I mean it bothers that individual terribly. Like for instance my being Transgender. I get so many people get to bagging on me, yet not a single one has ever met me. And some will persist for days to weeks to months and even years stalking through the internet just to cry out how terrible it is that I am Transgender. What creates a person like that?