Finally, there's scientific proof why we enjoy writing in our planners! It's all got to do with the feel-good neurotransmitter dopamine, "..the neurotransmitter that increases attention, optimism and determination."
This article on LifeHacker explains how you can improve your mood by making predictions, any predictions, because every time you make a prediction your brain releases dopamine. If you're right, you get that satisfaction. If you're wrong, you get a learning experience. Either way, your brain wins. The more predictions you make, the more dopamine your brain gets.
The article goes on to relate predictions to gaming, which I'm not disputing at all. But as you know, for me everything comes back around to planners. What are we doing when we write something in our planner? We are predicting what is going to happen. That meeting tomorrow, going to the gym, that deadline, after school activities: every time we write something in our planner, we are predicting that thing will happen. When we predict, our brain gets some dopamine.
So not only does writing in our planners help us feel organized and focused because we know what needs to be done, it also triggers the reward neurotransmitters in our brains. The more you use your planner, the better you'll feel!