Say someone has a big project and they think they it will be a bit hit and make them a living wage. Chances are, it won't. Their hopes are dashed, they get discouraged, they stop.
But say they didn't have any great expectations for the project, not expecting it to make a cent. But it does mildly well and earns them a bit of pocket money. Then they can be happy with what they end up with.
Additionally, say a person wants to find a significant other by the time they're 30. They put unnecessary pressure on themselves and feel they failed if it doesn't happen. Had the person have 0 expectations of finding anyone, then they wouldn't be any worse off.