Its OK to let you dog sniff other dogs - and totally natural and important to socialization

What It Means When Your Dog from Sniffs Down There by Kelly Modzelewski, Dog and Cat Writer

Some dogs are so affectionate that they're not content with licking your face. They also want to share your more private smells which can prove embarrassing for you or an unsuspecting visitor. Men and women alike are victims of this socially awkward behavior. Dogs do this out of a natural instinct to learn about this person (in the same way they sniff each others behinds) and out of habit. Keep in mind that dogs have a very strong sense of smell so they can learn a lot about a person through sniffing. This strong sense of smell also leads to another reason canines might sniff that private area - because some dogs can actually sense prostrate cancer. They can detect a problem just sniffing people but, according to a recent study, can actually pick up the scent of chemicals associated with early prostrate cancer in urine.