For many, like myself, who played its part of our blood. Its what I like to call the first "extreme sport". I dont know many other sports where u gladly sacrifice the health and well being of your body to play. ( except maybe rodeo bull riding or boxing)
When you are playing the game, its like a "high" that can be achieved by NOTHING else!
So, watching a game brings back lots of great memories.
Also, at least in rural America, Friday or Saturday nite high school football games were the main thing our social life revolved around. High School dating, drinking, drugs, sex, and rock n roll baby.
Again, watching a game brings back great mammaries.

And there are many subleties (sp?) that u may not understand about the game unless u played or grew up watching it.
anywho, thats my 2 cents