In the book The Happiness Hypothesis, Jonathan Haidt uses an analogy to explain the difference between the rational and the emotional system in our brains.

He says that our emotional side is like an Elephant and that the rational side is its Rider, perched atop the Elephant.

He then goes on to say that the rider might hold the reins and appear to be in control, but any time the elephant disagrees about which direction to go in, the rider is likely to lose.

The rider may be able to get his way temporarily by tugging on the reins hard enough, but he won’t be able to keep this up for long against a 6-ton beast.

In other words, going against your emotions, trying to remain completely rational, will exhaust a lot of willpower very quickly.