A Dog's Life

I am sometimes envious of dogs who seem to always just "live in the moment". What's past is past and the only thing important is what's happening right now. They can patiently wait for hours alone by themselves, then hop up to joyously run around and go pee, whenever the owner gets home. What do dogs dream about? Big meals, running in the woods, other dogs? When they are in pain, lonely, or sad, how do they just "suffer in silence".