Why Do Dogs Roll on Dead Animals and Poop?

[Ellie Sullivan]

My Norfolk Terrier mix does what we call "ectasy rolls". He finds something that smells good on the grass and rolls himself on it with a silly look on his face. We noticed he will dive and roll on other dog's poop as if it were a bed of flowers. Last weekend at the beach, he went headfirst into a dead seagull and started rubbing himself on it. Yuck!!!!!!! Why do they do this???
-Ellie