Riots at penn state

[Tellos Athenaios]

people are so completely wrapped up in 'their' team that it becomes their lifestyle, their defining identity.

Bingo. Sport serves one of the most basic psychological needs for many people: that of identity.

It also gives a convenient means to settle tensions in a somewhat civilised manner, as opposed to petty war. You could, in theory, have the entire Afghanistan war settle through a regular football cup, the psychological ingredients for it to happen are there.