The most important thing I ever learned... I have no idea. It's hard to think about your entire life and everything you've ever learned and pick one thing to be most important. Realistically it's probably something like "the world doesn't revolve around me" or "other people are real people too" or something else most people learn as toddlers in today's society. If I had to choose something that's both moderately important, and that I can write some sort of experience about, it's probably got something to do with making friends. Nothing specific, like how or why to do it, but just that the whole activity is generally a good thing.

There are so many ways people make connections with other people, like clinging to specific people and finding additions by chance, being excessively outgoing and forming hundreds of half-baked interactions, or even just being quiet and alone and coming across other quiet and alone people. However it happens, friends are important things to make and keep. As some famous ancient philosopher whose name I don't remember once said, "To live alone one must be a beast or a god", and considering how few people actually happily solo life it must be true.