For me, I think it was related to conflating my value as am employee to being able to efficiently and effectively complete tasks to a high standard in a short amount of time. It felt like if I wasn't producing enough tangible results fast enough, I wasn't doing a "good enough job". It didn't really matter if it was a day/week/month where I was absolutely crushing it, or a time period where I was having a hard time focusing; the end goal was the same. The internal satisfaction, if you can call it that, was accomplishing tasks. It wasn't (usually) to the point of overwork or burnout.
But the problem was, I never considered my own growth. It never felt like a good use of time to take on more challenging projects because I wouldn't get the same sense of accomplishment. It would be hard, I wouldn't have much to show for myself for a few weeks, and so on.
Obviously this is all subconscious, you don't realize you're choosing things you're good at, or an expert on. Expertise feels nice! And, to be fair, not everyone's employers give them the freedom to choose/learn/grow that many of us enjoy.