Finally an answer to the question..

Once upon a time all dogs were invited to a party. When they arrived at the party they were required to leave their butts at the door. All the dogs hung them on hooks by the door.

The party went on and the dogs were having a great time. All of a sudden a fire broke out. The dogs raced to the door and grabbed whichever butt they could grab first.

In an effort to find their own butt. That is why, to this very day, dogs sniff each other's butts!!!!