When you are young, and you are researching something as a PhD student, it really can seem like the most important thing in your life. All of your waking thoughts (and late night thoughts) can be shaped by it. When you walk outside you're not even noticing things around you but you're lost in thought about the current problem.

This is why it's so important to have some "spiritual" life of some sort as you cultivate an academic career, where you can practice appreciating the truly valuable aspects of life, and be grateful and peaceful when you need to be. For many people on this path you don't learn these things until much later in life. This is reinforced by your entire life being dictated by grades and exam scores until the moment you start your research.