There is no thinking they are similar. It is a widespread, wholesale problem. Over 1/3 of the sports fans that I know, have met, or have talked to share these insane tribal allegiances. It is evident on Spurstalk with the Bucks threads, people screaming at each other over who is best, and a general contempt for people who support different teams.
Why does it make you better to be a Spurs fan? This is the same concept that applies to ignorant nationalism. Why does it make you better to be an American, or make you proud that you are? In both cases you simply lucked out. You were born where you were born, and that is all you can say about it. You are products of your environment, and have no affiliation with the Spurs. You will never take center court for tip-off, you will never run to the locker room to share in the victory celebrations and post-game meetings. You are a fan. Plain and simple.
People need to understand this issue. It is, as I said, directly in coorelation with nationalism and its practices, and nationalism is the downfall of modern society.