Part of the canine greeting is to sniff and lick faces. Because people are so much bigger than they are, dogs try to bridge the gap by jumping up. Some people really dislike it when dogs, their own or that of someone else, jump on them. But dogs, on the other hand, do it all the time because they think they are being polite.
The next time you take your dog to the park, watch how he greets and is greeted by other dogs. They start by sniffing each others mouths, then move around to sniff the back ends. All of this is considered proper and polite behavior among dogs, and they assume it is the way they should greet people.
Dogs do not have any trouble sniffing our bottoms, they will do it all the time if given a chance. But the first part of the greeting, the mouth sniff, is not possible because we are so much taller than they are. Except for very short people with very tall dogs, the only way this is going to happen is if dogs get up on their hind legs.