Not to put too fine a point on it, but your typical football (soccer) supporter is a crassly self-absorbed zealot. We speak through the veil of a love for our club and the "boys on the pitch" but in reality the bulk of our mythologies - whether grounded in real events or not - essentially amount to tales of debauchery. This isn't to say we don't have passion for the game - for many of us, in fact, it is all-consuming - but what happens to a supporter is not unlike what happens to a person who joins certain churches: what begins as an earnest attempt to find expression and fellowship for a deeply-held belief can end up detrimentally altering that person's worldview. What you end up with is someone so indoctrinated that they lose the ability to step out of themselves and observe their own irrationality.
What starts as a high-minded endeavor often deteriorates into a sort of lowest common denominator groupthink, the kind that will willingly overlook the dogged efforts of a team forced to play a man short against an elite opponent because they would rather boo the referee for his perceived inadequacies, if not his outright alleged bias against your club and your city and your people just because the hands of fate might have dealt you a lesser hand to play with.
We see this mentality a lot, not just in religion and sports but also certainly in politics and really all of sociology. Invariably, you will find the staunchest believers to eagerly defend it - I'm even one of them myself. That doesn't change what's really going on, though. Supporting is a largely thoughtless exercise. Of course, there is a lot of thought that goes into putting it all together, but that tends to happen quietly within the strict hierarchy which governs such groups. As I have explained to many people, supporter culture offers a weird mix of high-school cliquishness and Italian mafioso tradition. We love to consider ourselves as a certain type of outlaw class, which stands in stark contrast to the close association most groups have to the front offices they often rail against.
All of this seems like a pretty lousy way to explain why I am a supporter. But you have to understand that I have become comfortable with the fallibility of human social constructions. For everything in your life that you love, you are forced to deal with things you don't really like, and yet we all choose - consciously or subconsciously - to love what we love about it, and overlook what we don't.
It isn't really about the capos, or songs; the banners, flags, 2-poles, overheads, or even smoke and flares; or even the supporters themselves. Of course, these are all great things, don't get me wrong.
Everyone has their own reasons for doing it, and I can only speak for myself. I have had a life-long obsession with this silly little game of soccer. Player, aspiring coach, student of the game: that is why I chose to be a supporter. Everything else, quite frankly, is peripheral.