It's fundamental because it's the primordial experience of self. It's the first thing we come to believe. I exist, and I am important. In Buddhism, this is self-grasping and self-cherishing, the root of all suffering. It's also considered fundamental because if you can accept that the self doesn't exist intrinsically, the self is empty, then all the struggles related to self evaporate. You don't prop up the self with pride. You don't denigrate the self with loathing. About 80% of your thinking goes away because you don't think about yourself at all. When you deeply acknowledge everything you believe about yourself is an illusion, you don't find much reason to think about yourself at all. All our sufferings are based on the self, and if it's completely abandoned, all our suffering is abandoned too.

Now if I could just figure out how to do that....