I consider to be really demotivating that you work really hard on an idea or project, alone or in a team, and in the end the project dies or fails because of external reasons.
In a work environment, those external reasons could be wrong decision taking by upper management or founders. Like when they try to achieve so many things at once or don't focus on what's really important.
That's something really demotivating and could be more if you'd like the project you're or were building.
In the end, this is why it's important to understand how MVPs work, how they help removing ambiguity from software projects, and know how to properly fight or educate company clients on what's best for them and their project.