Well dogs do this all the time. Everyday, multiple times a day. They love it. Dogs can't wait to sniff another butt. If your dog had money, he'd probably pay you to let him sniff yours. Kinky, I know, but really it's just their way of saying hi. But if you did that you'd get punched, slapped, yelled at, or tossed in a padded room.
(flicker photo Arse sniffers by Ross Angus; http://www.flickr.com/photos/ross_angus/767911035/)