When dogs say howdy, we dogs focus not on the face but on the other end - where all the profile info is. Dogs, in fact, would never have invented Facebook. We would have invented Buttbook.
A mutual butt sniff is the equivalent of a human handshake, I suppose. But much more polite... and informative.
People are terribly squeamish about butt sniffing. I have even seen dog owners at the dog park berate their dogs who are following dog etiquette with dignity and grace. Without butt sniffing, dog parks would be war zones.
We aren't the only ones who find butt sniffing the best way to get acquainted.
(That dog is not me - even though it looks remarkably like me. We don't have javelinas at the dog parks in Minnesota, to the best of my knowledge.)
To think about dog butt sniffing in another way (and laugh hysterically, if you share my Susan's sense of humor), check out Creature Comforts America: Season One, Episode One in which two men discussing wine become two dogs sniffing another dog's rear.
But whatever you think about butt sniffing, remember: dog olfactory bulbs are forty times bigger than humans', relative to brain size. So who's to judge?