What also still puzzles me about habits is the fact that once I have a routine (like going to the gym at 6am) it is super easy. But once I lost it I struggle a lot to get it back.
The paper might explain why it is so. Routine-setting in your brain runs separately than the execution of the activity itself, meaning you need to train the two separately, at least from what I understand.
I feel like it's much the same for bad habits broken, too. If you allow yourself to fall back into a bad habit (smoking, drinking, gambling, etc for example) it feels really challenging to break it yet again. Best excuse I suppose to keep those good habits and bad ones right where you wanna keep 'em.
When I start a habit it feels like a one-way trip to a new life. If I break it, even once, I realize it’s not, it’s just a thing that I can do or not do. Now starting that habit again feels much less consequential.