What is a friend (and how does that differ from a close friend)?

Frankly, I don’t even talk about mental health with my wife, and I would say she is my best friend. I imagine this is cultural, but I’m not sure what that means. It’s not like anyone ever said to me, hey, never talk to your friend in private about mental health. And that isn’t something I’d expect people to talk about in public with each other where I could learn from observation. In other words, it doesn’t feel like something that is learned from explicit teaching or from observation. So, where is it learned from?