Dog's sniff the ground because smelling is their favorite scent and the ground is covered with good smells, like foot prints and piss. We don't sniff the ground because smelling doesn't do anything for us, and because the ground is covered with foot prints and piss. If you see a dog sniffing the ground, he or she is figuring shit out, getting places, accomplishing things, and living it up. If you see a human sniffing the ground, they've escaped from the local mental health institution, and they need the immediate attention of a psychiatrist...or they're a werewolf.
(flickr photo Bloodhound Trials Feb 2008 -154 by Contadini; http://www.flickr.com/photos/92305862@N00/2307362444/)