Why do dogs do this?????

It all about natural instinct. Dogs will roll in dead things to hide scents, roll in the grass to hide scents, kill small game for the sport or to actually eat, bury their kill, flount and show you their kill for pride, eat poop from almost any animal, roll in poop, eat dirt, sniff asses, sniff everything, etc, etc, etc. What is funny is that we are sickened by what a dog does from time to time, but no matter how sick and twisted and evil their masters act, dogs are still happy to see their masters whenever they return. What is the lesson?

Dog's are incapible of teaching good morals.