In the past, when women where kept isolated from men, and close contact between the two discouraged, men were more likely to be intimate (not necessarily in a sexual way) with one another. It filled a need for human contact. I'd be willing to bet that since rules about physical touching between men and women have relaxed, they have become more exaggerated between men.

A lot of it is cultural, too. When my brother and I greet each other after a long absence we kiss on the lips, although we have learned never to do so in front of strangers.

American men, in particular, seem extremely awkward about physical proximity to other men (compared to other cultures), and they don't even seem to be aware they are that way. Even in normal conversation, they appear very uncomfortable to approach each other any closer than about 18 inches. Try it it sometimes. Its amazing what lengths people will go to avoid having others violate their space. And they don't even realize they are doing it.

Cultural rules about zones of avoidance are just that, taught and learned. And they change over time.

The football player's congratulatory slap on the ass is only allowed because they are so sure everyone sees them as tough, macho guys, and no one would dare suggest, even in jest, that they were acting in an unmanly fashion..