I think that people today tend to overestimate motivation, and underestimate routine. When you say "motivation" you mean emotional energy that push you to do things. When you say "routine" you mean mechanical habits that keep you doing things.
But emotions are not stable, they come and go, and you need to feed yourself with similar emotions to keep your motivation. On the other hand, routines are more consistent, you are tied to your habits and you don't need efforts to keep them, once established.
That's why you need motivation to lose weight, and it's hard ; but you just let your routine drive you to get fat, and it's easy. Now, I used an example where routine leads to a bad thing, but the real challenge is to find : how to find and establish the effortless routine that will lead me to a great achievement?
Please note that I didn't say self-discipline but routine. Because self-discipline is also hard to maintain, it has more to do with motivation, than with routine. Self-discipline needs a constant effort to rule yourself, but routine is effortless, you just let go.
OK, now let's finish with talking about the hormones of motivation, which are dopamine/endorphin to give pleasure, and adrenalin/cortisol to give fear. I think that motivation needs a big load of dopamine/endorphin/adrenalin/cortisol, that's why it's hard to keep in the long run. Not like routine which just needs a minimum quantity of these hormones, that's why it's effortless.
People prefer motivation because they like big emotional fluctuations, they want their inner ocean to have big waves. And that's why they reject routine which has less emotional fluctuations, they are bored when their inner ocean is stable and quiet.