Why do Dogs sniff eachothers back ends?

Because

Google has some great answers for this.

And there’s always this stupid answer…

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, to this very day, dogs sniff each other’s butts!

Because they think they smell good?

They gain a lot of information by smelling each other’s buttholes. In fact, they gain a lot of information through smelling everything. Their power of scent is many times higher than a human’s power of scent.

Because they can get away with it.

They can’t shake hands, so they gotta do something!

Recognition. Everyone has their own distinct scent. A dog’s sense of smell is strong enough to make the distinction.