He who fortifies himself completely against boredom fortifies himself against himself too. He will never drink the most powerful elixir from his own innermost spring. — Friedrich Nietzsche
Our brains were never wired to be truly rational, because there is way too much information in the world for us to process. So we evolved to selectively process information to make decisions quickly.
When you procrastinate, you opt for the instant gratification of enjoying yourself rather than the future reward of accomplishing the things you set out to do.