From Atomic Habits, by James Clear:
The greatest threat to success is not failure but boredom. As habits become routine, they become less interesting and less satisfying. We get bored. Anyone can work hard when they feel motivated. It’s the ability to keep going when work isn’t exciting that makes the difference. Professionals stick to the schedule; amateurs let life get in the way.
The only way to become excellent is to be endlessly fascinated by doing the same thing over and over. You have to fall in love with boredom.