I think getting older is part of it. I used to love learning about different assembly architectures and spent lots of time on it, but as I got older a couple things happened: one, other things take up more of my time. Two, I already know a lot about assembly, so I don't get much from learning a new architecture that is probably only marginally different from one I already know.

I remember discovering HN around 2008 when I was a teenager. I hardly understood anything on the front page. I knew how to program a little bit, but only at a superficial level. Now, it's rare for me to come across something I don't understand unless it is very specialized or about some scientific topic. But I think Slava is right that if you want to stay motivated and stay interested you have to find a way to remind yourself of the bigger picture. Otherwise you get bored no matter how interesting the work is. I know a guy who left a job working on rocket guidance systems because after a few years it got boring to him.