There are three main reasons why dogs chew:
* Chewing is a natural desire. It's fun, it passes the time, and it's a self-rewarding, self-reinforcing activity. As example would be chewing on something that tastes good.
* Chewing is an outlet -- especially for a nervous, bored, or lonely dog. The repetitive act of chewing is actually quite soothing to your dog. It's the doggie equivalent of comfort food.
* Chewing burns up nervous energy and gives your dog something to do. This is especially true in the case of dogs who don't get enough exercise.