When I was at one of these many courses the jobcentre sent me too, there was this guy who had my current obsession (a thing I was obsessed with at the time). I was so jealous that I hated him. When he sat beside me, I'd turn away or even move away if there was space somewhere else. If he tried to talk to me, I'd ignore him or give him a dirty look.
I was actually quite ill at the time, with so much intense jealousy bubbling inside me. I couldn't wait until the course was over so I never had to see him again. Obviously, I couldn't wait for it to end anyway because it was awful.
Then a few years later, I was sent on another course and he was there too. But by then, that obsession had ended and I had moved onto a new one. But it turned out he didn't have that thing anymore, so there was no reason to be jealous anymore.
I felt really bad for hating him, over something so stupid. I think he remembered because he avoided me, at least at first. After a 2 or 3 weeks he did talk to me and I talked back. But I always felt bad about it. These obsessions are unhealthy.