A butt sniff for dogs is like a handshake for humans. called the Jacobson's organ that lets them ignore the smell of poop when they sniff another dog's rear.
They don't just smell butts--they live and breath butts.
Have you notice a fish like smell coming from your dog? your dog; The famous dogie butt scooting; Red swelling in the anal area; Dog nipping at the anal area.