why do dogs sniff butts?

It's their way of greeting/ communication with other dogs. They find out information of that dog based on its smell, e.g. it's gender, reproductive status, it's emotional state etc. It's the dog version of shaking hands with someone and introducing yourself :)

It seems gross. And it's embarrassing. But the gesture is a common (if poorly mannered) canine version of the "getting to know you" routine.

Dogs often greet one another by sniffing — which makes sense when your ability to smell is 10,000 times better than a human's. We all get that. But why does it have to be the butt, you ask?

Well, even the least smell-sensitive human can tell you that’s where a dog’s most intense aromas reside. In fact, there’s a pair of anal glands back there that secrete perhaps the most foul-smelling stuff you might occasionally have the displeasure to experience. So it’s perfectly normal.

Problem is, dogs often engage in this behavior enthusiastically, which can get them into trouble with other dogs who’d rather not have their butts smell tested. And, of course, it can also get them into trouble with humans who’d prefer that a dog not get so fresh with them. If your dog is an aggressive sniffer, it is something you can take up with a certified animal trainer or behaviorist.

Dogs have a pair of musk glands in their anus, and the muk gives other dogs information about their staus, health, diet and so on. thats what they are smelling.

for the same reasons humans shake hands