A million things. Of course, there's no hard date such that when you turn 40 then everything changes. It's just a rough age range.

I'll answer your question by repeating the most common things I've heard from others, not my own experience, in an attempt to reduce the error band by not having a sample size of 1.

By your 40s, you generally have at least a direction you're going. People tend to take you more seriously. You have a greater sense of what's actually important and what's not, and stop wasting energy getting as worked up by things that don't really matter. You are still young enough that you aren't yet battling many adverse effects of aging.

You also tend to be at your most physically attractive point. Your career path is likely to be established and less uncertain. Romantic partners tend to be more stable, so your love life brings more joy. Speaking of romantic partners... relationships are better because (as a friend of mine once put it) "sex isn't such a fucking emergency anymore". Your relationships tend to be more balanced. And so forth.

I think, really, it's an age where you have gained more perspective and are still young enough to use that to live a better life.

The ‘fuck it’ bucket. You realize that life is here and you stop caring about the social constructs you’ve been part of. Also, while you aren’t your virulent former self, you can do things well OR you can dive into something with the curiosity of a kid knowing you don’t need to be great.