Why is going to the potty so hard?

I ask the question because the Boy knows how to use the bathroom and does a good job except when his body is telling him it is time to go. I realize sometimes excitement gets the better of kids but he can just is determined not to go sometimes and it gets to be issue when we are going to be in the car for a bit or going to bed. Last night for example as he was asleep he needed to go so I got up and got him to the bathroom as I didn't want to change his whole bed. Then later in t he night he had to get up and drop a few friends off at the pond. I don't really get why he just won't go as it not only makes it hard for us to sleep and makes him tired and grumpy in the morning. My favorite is "I don't have to go!" and then it sounds like Niagara Falls as he is crying saying he doesn't have to go. Independence is important to kids but our house would be much calmer if he would listen to his body as he gets more and more crazy the longer he waits. I just don't get it.