A Dog's Normal Life Cycle
If a dog isn't actively engaged in some activity, which can either result in an interest in certain objects or just naturally moving due to being energetic, he'll just lie down (as a lack of stimulation prevents him from moving around). This is the regular activity-rest cycle of a dog's day to day life; dogs nap because it's what comes naturally.
Make Your Dog's Naps More Productive
Napping is healthy and normal, and it serves a purpose, of course. The body rests, and as a result it optimizes the immune system and functioning of the organs. That being said, it's a good idea to help your dog nap as comfortably as he can. It's nice if a dog has a bad to nap on-that way he has his own comfortable space and gets used to a familiar smell and even the texture of the material fabric.