Football is typically held up as the quintessential American sport in many regions of the country. Those who don't proclaim their interest or love for it do so at the risk of being seen as outsiders and potentially treated as outcasts.
This starts at a very early age for many Americans due to the close ties between the education systems and football programs. Starting while they're children, those who are capable of playing are often expected to play, and those who aren't capable are expected to participate as fans. This continues through to high school, college, and to the NFL.
Even if the fans don't particularly like the game itself, they often like the social aspect of it, or just pretend to like to to avoid being stigmatized.
Association football (soccer) and cricket play a similar role in other countries. They can be seen as a form of religion, in some sense.