Ask HN: Why shouldn't we rely ultimately in motivation when doing any activity?

There are some discussions about the influence of motivation when doing any activity and how we shouldn't rely on it for long-term efforts. For example, coding a SaaS project could be exciting during the first days, but stop being fun after some months or when you have to work on the marketing aspect. The advice given for that is usually that one can't rely on motivation to do something in the long term and we have to endure through the process. But I'm sceptical about this. Shouldn't be striving to find back the motivation instead of pushing ourselves for the sake of the project? In a full-time job, the motivation is, in the last instance, money, and if at some point that motivation doesn't work anymore we look for another job. The advice found in these cases is not endure in that work and push ourselves doing that job. I'm curious about the dichotomy between the motivation in entrepreneurship and side hustles versus the motivation in the ordinary full-time job. Does the fact that our side hustles are something personal justify that we should endure and not rely only on motivation? Because side hustles are personal projects, shouldn't we be motivated and find that motivation to do it? And otherwise, move on to something that we are more interested in at that point. I'd love to hear your thoughts and hope I made any sense.