There are some crucial things that can really alter the course of a friendship. The first big one that I've noticed, maybe you've noticed this as well, is distance. Something that becomes so much more commonplace the deeper we get into our 20s is that people move away from the places where you once met and settled. It's not necessarily just distance that causes you to grow apart but other more nuanced factors. Some of my best friends, my closest friends are people who I rarely see in person. We still share so many of the same values and have that unconditional love and support for each other.