It's just a "way to go" type of thing. As long as you're not gay/homophobic, it's really not a big deal. I don't do it, but I don't see the problem either.
Why do Presidents slap female athletes butts? <img src="http://latimesblogs.latimes.com/photos/uncategorized/2008/08/09/beachvolleyball2.jpg">
Nothing gets your adrenaline pumping more than a erotic tap on your ass. It also sends the helpful message, "Hey if you start sucking, I'm gonna rape you in the locker room."
Because Brett Favre does it. Something only true American good ol' boys do, like wearing Wranglers and sexually harassing massage therapists.
As a fellow teammate is leaving, it is naturally the part of the anatomy that is the one that will absorb the most upon an open-handed hit and also at the height of the hand. Having a teammate whom you trust and you let hit you on the butt is possibly a ritual that adds to the cohesion of the group. That's my opinion and why it seems logical to me. I believe it has NOTHING to do with Amigo.
