I saw someone post this article the other day and thought it was pretty interesting:
According to a recent study, dogs are much more excited to receive a treat when they earn them vs. getting them for no reason. This specific study trained dogs in various problem solving tasks and then measured the level of excitement by such things as wagging tails.
It is pretty interesting that dogs (similar to humans) enjoy rewards from accomplishment more than from doing nothing. I guess the results make sense given many dogs enjoy working, but I am a bit surprised they like treats more after performing a task rather than just getting the enjoyment out of the actual task.