Two Sides to Every Fan

So there I was sitting in the bar after the AFC Wimbledon game watching the Arsenal game on the big screen and I glance across the room to see a kid in an AFC Wimbledon shirt cheering for Arsenal! WTF?? Come on kid, you come here every home game, what the hell are you doing cheering the gooner scum? Another thing, if you're from South London and you go to football matches in South London, why are you cheering a team from North London?
It does remind me of my best friend though who supports AFC Wimbledon and also 'supports' a premiership side, what the hell is this phenomena about, surely you can only have one team, can't you?
Personally, as you should know by now, I am a Dons fan, I do look out for Portsmouth scores too as my Grandad took me to my first ever football match at the age of about 6 and it was Portsmouth v Manchester City at Fratton Park but I wouldn't call myself a Pompey fan.
Any explanations folks? I really don't understand this need to support a top flight team if your team isn't in the top flight, is it an acceptance thing? A need to be liked? A need to feel popular?
Help me understand.