- Proximity of class, demographic, religion, ideas or other characteristics.
- Psychology -- if one grew up in a Muhammadiyyah, NU, Sukarnoist or Peronist family or neighborhood, then most likely I will vote for PAN, PKB, Megawati or Kischner.
- Rational choice -- a voter calculates the cost vs. (expected) benefit of giving his/her vote, and use the act of voting as a strategic behavior (reward or punishment).
Clearly, the rational choice model doesn't apply very well here. I don't know what I could gain by sticking to them, except for pure satisfaction.
Nor does the proximity model. I am not from Merseyside, so I don' support them based on regional identity (unlike my support for Persib Bandung). And the team doesn't resemble any class, religion or any identities. Some teams may still do -- look at the rivalry between Lazio and AS Roma; Glasgow Celtic vs. Rangers; Napoli or any Sicilian teams vs. the Northern Italian teams.
That leaves me with only the second explanation: psychology. It may be true. I started watching football when I was a kid, they were the strongest English team, and one of the strongest in Europe. Being a new comer, naturally I picked the current champion at that time (my family was not really a football lover, so I didn't inherit the choice from my dad or anyone). And the choice remains the same until now.
Hey, that's the different between fans and supporters, isn't it?