Both of you are correct.
There is certainly a level of glory hunting attached to this, however, IMO, most OF fans support is down to bigotry. If they're not bigoted themselves (most of them are on some level) then it's down to 'I support (insert appropriate arse cheek) because my father did and so did his' blah blah. They're ancestors support was originally down to bigotry. If everyone in Glasgow supported their local team, Arse cheeks, Partick & QP would surely all have a similar number of fans?
There's peer pressure as well of course; if everyone in your class supports either one when you're 8 years old then chances are that you will too. Again though, the fact that everyone in the class supports them is originally down to the bigotry thing. I could well have ended up supporting them too if it wasn't for my father, who had no interest in football, from stopping me.