Why does my dog lick her feet all the time?

The first thing that comes to mind is that she could have an allergy. Dogs can be allergic to food, the surface they walk on, your cleaning products, even things floating in the air. Licking her feet leads me to believe it could be the surface she's walking on. Check your cleaners to make sure they are "green."  If you have grass outdoors, don't use any insecticides or growth products.  Check your dog's paws to make sure there are no foreign objects stuck there. Have her examined by the vet to help determine the cause of licking.

Just to back up what Mamairis said... My dog used to do the same thing. He would lick his paw pads until they bled. We found out it was a food allergy, and the vet put him on a lot of expensive meds that only helped some. After doing some reading, we just started making his food. That made a huge difference, and he didn't have to take all the meds. His ears cleared up too.