It smells good.
It gives a lonely dog something to do.
It makes a nice cool hole where the dog can lie.
Tunneling to make an escape under a fence.
The dog hears something down there and is going to dig until he finds it.
Cosmo was never been a big digger, just the occasional superficial attempt to make a hole — just because. Clover, on the other hand, was never a big digger until she met my parents’ dog, and now if left alone in the yard for “too long” she will start her attempt to turn the yard into something akin to the surface of the moon. Clover used to dig in order to remove something: a tree, a rose bush, a flower or weed. Now, if given the chance, she will dig just because she can. Note to my mom: Clover was NOT being spiteful, she was just having some fun.
One of our dogs from long ago, Kelly, dug halfway to China once. There is a photo somewhere of the hole about 3′ x 2′ and so deep that the only thing visible was the tip of Kelly’s tail sticking out of the hole. Kelly liked to dig more than any other dog I have ever known. From the same era, Kelly’s sister, Casey liked to dig, but with a purpose! She was after moles or voles or anything that was beneath her feet. I still shudder at the thought of her little (dead) presents.
I do have another theory. Clover and Cosmo are looking for the Nylabone that Casey and Kelly buried out in the backyard about 15 years ago. Its out there, pups. Go find it!