To Them, the World Is All About Power and Control
Take a step back and think about the world from your toddler's perspective. They have little say in what they do, what they eat, where they go, etc. Parents have all of power and control. So what can a child control? The clothes they'll put on, the food they'll actually eat, if and when they'll use the toilet, when they'll fall asleep, and what they can and will say — the areas where parents tend to have the most trouble with their kids.
As Dr. Klein writes, "I guarantee you that if you view parenting as a power game, with you as the parent truing to assert control over your child, then you will have to keep upping the power to win. And the child, over time, will continue to battle, fighting back as he tries to show you with greater and greater force who he is, what he can do, what he needs from you."
Source: Flickr user pinksherbet