What a great post and it's a good question. Over the years I've wondered this too and figured I also couldn't be like that. I'm still not that way but I have had glimpses recently.

For many years, sometimes consciously sometimes not, I'd resent those types of people because I wanted to be like that deep down. I'd get frustrated about it and it would compound itself.
I find when I really let go, admit I don't know much of anything, and really admit that I don't have control over much of anything beyond myself, I feel and act like the guy you're describing.

The thing is though you can too. You're just telling yourself you can't for whatever reasons you tell yourself. I do it too and have for years. It's difficult to break.

He tried to convey this to you when he said, "we don't have control in our lives so we shouldn't feel so down."
There is a lot of wisdom in that, but on the surface it sounds so simplistic, maybe even trite to those who are unaware.
If we're stuck inside ourselves it might as well seem like a foreign language. That could be why you've heard it a lot but maybe you aren't getting it.

Making the post though was a good idea. Be Still has a good interpretation too. I would check into practicing mindfulness for yourself on a regular basis. Best of luck in your day today!