I think it's in your blood, or at least it's formatted into your system at such an early age you're "claimed".
I can't say I follow Celtic anymore, it's such a bloody bore, as the SPL has become a procession, and these days with so many foreign players in the Scottish game I can't say I care any more about Celtic than I do about Arsenal or Chelsea.
It must be more fun to support the "smaller" clubs because their fans must surely identify with them more.
I've always followed the Celtic/Man United axis, probably because they won the European Cup one year after each other, and I would have been 11 years old then, just the right age to notice.
I started going to Rugby Park (Kilmarnock's ground, for those who don't follow football) quite a bit when I lived in Irvine, they were the nearest and biggest club, but I also went to Hamilton Accies, Motherwell and Ayr at least once, though the grounds were shocking and the atmosphere was akin to a bus terminal at midnight.
When my dad was posted to Glasgow, I lived in Maryhill and was able to sneak into Firhill quite often. I still have a lot of affection for Partick Thistle.
Over here in Oz, it's as though the game has only just started out. For many, many years Australian football was riven with stupid ethnic factionalism (Croats v Serbs, Macedonians v Greeks, Italians v Croats, etc..) stuff which Australians see as definitely un-Australian, and the game was too easily painted as "wogball", i.e. the game that dumb immigrants and poms played.
Lately, Australian football has had a severe makeover. The failed old NSL (National SOCCER League) has been crushed by very astute businessmen who want to see the World Game take a solid hold in Australia, and since the death of Johnny Warren, a very popular football commentator who hated the game being known as "soccer", the country has come round to calling it "football", and not before time.
The launch of the new "A" League a few years ago meant that a number of national teams were created from scratch, one in each major locale, perhaps too clinically, too off-the-peg. As such, the support for each club is quite calculated and the supporters naive, with the family aspect being promoted at grass roots level.
I suppose it's a start for the game here, and more importantly the game has been taken out of the hands of the ethnic incompetents, who dominated it in the past, and put on a financially sustainable footing, but I do miss the grime, the grit and the hilarious banter on the terracing.