Why does my dog keep licking herself?

I would add just two more possibilities to the list that Deb gave. If your dog is licking his/her rear end, it could be a sign that he/she needs the anal glands expressed or there is a urinary tract infection present. If you notice discoloration in the fur, skin irritation or the licking is non-stop, no matter the area of the body, you should see your vet.

Some licking is just a part of natural grooming. However, when it becomes excessive, it may be a sign of allergies (environmental -i.e. mold, pollen, dander, cigarette smoke, flea bites or food-based), other irritations (hot spots, etc.) or a source of pain (i.e. embedded foreign object such as bee stinger, etc.) Another possibility is boredom, anxiety tension, stress release.