From a letter I wrote to a friend:
I hope we can stay in touch going forward– I think old friends are important in life. It’s so easy to believe that as time passes and as we experience seemingly epic and life-altering successes and setbacks and dramas, we are changing so much. Abandoning our principals, our passions, our personality. But the people who’ve known us forever see that we’re still essentially the same underneath all the superficial changes that time wreaks on us. I don’t know what identity is or believe that the “essence” of a person exists, but somehow, old friends help to elucidate it, or maybe bring it into being, and in a confusing mercurial world (or, in my confusing mercurial world) there is something anchoring about that.
There are two corollaries to this idea:
1. What is anchoring can also be stifling and constricting, but in society as Gesellschaft it’s the stability and anchorage we lack and want and seek.
2. When we lose a friend, we lose a piece of our identity. That is why breakups and separations of all kinds are so difficult.
For more on friendship, read Aristotle’s Nicomachean Ethics. Also this Yale philosophy course (from way before EdX was trendy) has some thought-provoking digressions into the philosophy of identity.