I learn about something when I need to know in order to do something I'm trying to do, or when I'm curious. The second one is kind of self-limiting - when I don't have time or energy, I'm less curious.

I don't feel that I "ought" to know. There's too much. I can't. So I learn what I need, and what I want.

Left behind? You have no choice. When you're learning A, you're not learning B through Z, except that there are a lot more than 26 options. You can't learn it all. There's simply not enough time.

The real question is, of the limited amount of time you have for learning, what's the most important/valuable thing for you to be learning now?