If I get used to being physically active, the next day that I am NOT physically active I feel terrible.
On the other hand, if I have been lazy for a period, then one day I do a very brief exercise, even a 10-min moderate run, makes me feel excellent. However when I start doing those runs everyday, I don't feel that same good feeling.
How hard are you pushing it? Increase the difficulty/duration, and you may find a different outcome. I'm not talking pro-athlete level, but competitive high school athlete level (2-3 hours / day, 5-6 days / week, with some good lung heaving moments each day).
> On the other hand, if I have been lazy for a period, then one day I do a very brief exercise, even a 10-min moderate run, makes me feel excellent. However when I start doing those runs everyday, I don't feel that same good feeling.
I don't think this is the opposite. One is moderation of laziness, the other of physical activity. "If I'm lazy for a week, the day of exercise after that lazy week is more enjoyable".
* If I don't eat sugar for a week, the next time I eat it, it tastes way better
* If I don't watch TV for a week, the next time I watch a good movie, it's way more enjoyable
* If I'm physically active for a week, the lazy day after that week of physical activity is more enjoyable
In my experience, a lot of things in life seem more enjoyable when moderated. The list can certainly be expanded.
No clue if this is official dopamine fasting, but that's what it makes me think of. And to be honest, I don't care what the science says on this*... my own personal experience is important enough for me to change the way I behave.
* unless it says it'll somehow lead to some horrible outcome down the road... then I'd consider it