Ask HN: How not to tire of a personal project?

I have read several stories about personal projects turning into something big or at least big enough to make a living out of it.

Working an hour or two every day would take months to have a minimum viable product.

Were people who completed a large-scale project simply having a great deal of discipline and did not get tired of the project? If so, how to develop such discipline?

Or did they have the opportunity to quit their job to accelerate development?