Dogs think and make decisions every second. What they are thinking of depends mostly on their environment and what they're already doing at the time. However, the most common instance where you're probably wondering what your dog is thinking is when they're looking at you, at home, and the answer would be:
"What's happening next?"
That's all there's to it! Dogs have become mankind's best friend partly because they're ready and happy to do things with us, at all times. There are of course exceptions to the rule if you can think of dogs that would rather spend most of their time napping, but as you can imagine most dogs will get up from their bed, wag their tail and jump the moment you grab their leash to go for a walk thinking of what will happen next.