One of the essential features of ADHD is the rapid attenuation of the reward system, leading to a biological resistance to the "dopamine rush" that neurotypical people feel. (For me, it manifests most clearly in the fact that I have never in my life felt anything like the "runner's high" after exercise, although every neurotypical person I've spoken to says they feel refreshed, rejuvinated and pleasantly tired afterwards.)
This stems from the fact that the built-in reward system (the positive emotional response to performing/completing a task) attenuates very quickly in people with ADHD. By that I mean that while the response happens, it very quickly drops back to zero. Much faster than for people without ADHD.
This, I suspect, is one of the fundamental aspects of ADHD and why it's characterized by attention deficit and hyperactivity. Hyperactivity happens because in order to maintain the effects of the reward system we have to do and do and go go go over and over and over again. And we have attention deficit because our interest in any given thing drops extremely quickly, since the reward of experiencing it goes away almost immediately.