My interest has mainly been trains, although I've gone through phases of being into buses, so I've experienced enthusiasm in several spheres. I think it's very true that autistic spectrum disorders are often the drivers for some of the more weirdly obsessive behaviours in the enthusiast community, but also some things come out of being young. I used to be a keen spotter as a teen and twentysomething, but I haven't felt the need to collect numbers for years, and my interest in most forms of transport is now driven more by my enjoyment of travel than any rivet-counting obsessions.
I think, like most things, that the degree you're drawn in and the level it takes over your life are the significant factors as to whether you harm yourself or others, if that makes sense. If you have a well-rounded life with plenty of interests and a good social life, no-one will bat an eyelid if you have a soft spot for a particular bus. If it becomes all-consuming and excludes everything else, such as a normal social life, or familiarity with soap, then that's a problem. I do have a tendency to take things very seriously, which was a problem when I was younger, but it's much less of an issue as I career chaotically towards middle age, and I'd like to think I'm pretty well-rounded.
Something that has always bothered me, though, is that if you're obsessive about football, no-one will bat an eyelid, but an equivalent level of obsession about practically anything else will get you labelled as some sort of weirdo.