I used to think that the core motivation of a good founder needed to be a mission they believed in.
But if you look, there are many companies that start with one mission and pivot to a completely new one. For example, Slack started as a gaming company and then became a messaging app. YouTube started as a dating website and then became a video hosting service. If the core motivations of their founders was a mission, how did they change so much? It seems that, at least at the beginning, these founders were motivated by building a company at all.
The best motivations will keep a founder going no matter what obstacle is in their way. This means there are good and bad motivators, as some motivators won't stand the test of large enough obstacles. Good motivators are sustainable, while bad motivators eventually hit a wall.
Good founder motivations are things which 1) The founder really wants, and 2) the founder can only get by building their business. These could include things such as 1) a strong personal attachment to a specific mission (e.g saving the climate, or ending poverty), 2) never wanting to work for someone else, or 3) a genuine interest in the problem they're solving (regardless of impact).
In contrast, bad founder motivations are ones which 1) founders don't want enough, or 2) they can get more easily than by building a business. For example, it's a lot easier to become moderately wealthy by getting a high paying job and making good investments over 10 years than by building a company.