As you see when you have more than one dog, or see many dogs at a park, they travel in packs and use smell to identify each other (which explains why they sniff each other's rears, as well). Dogs also see humans as part of their pack, therefore they want to be able to identify them in the same way -- thus, the reason dogs sniff crotches (and also because this is usually the closest spot to their nose that has a human's identifying scent). In addition, dogs just generally like to sniff interesting things...garbage, flowers, people, you name it - they sniff it :-)Source:
"No Bad Dogs" by Barbara Woodhouse
Photo Credit:
kalimistuk -- Flickr Creative Commons
