Mud: Why do dogs love it?

I think it differs from breed to breed. I used to have a retriever who was just the same as the one in the photo. She absolutely loved mud and it was impossible to keep her out of it. Luckily she loved being hosed down afterwards too. I never did manage to keep the car clean though after she's been wallowing.

The dog I have now, doesn't go near mud, be she loves dirty water. She loves to roll in poo too. Not pleasant!