If the occasional person is thrifty, there's no problem.
If everyone is collectively "too thrifty", then we've got a problem, but this isn't the cause of the problem, it's a symptom. What does it mean when everyone is saving? It means that there is some kind of uncertainty or fear which is making people forego current consumption in favor of future consumption. This isn't a natural state of human affairs, since we tend to value things today more than things tomorrow. The extra "thrift" happens because of political or economic uncertainty.
Political uncertainty is fickle, you can't predict it or plan for it. However, saving is the correct response to economic uncertainty, and it's what's responsible for the eventual recovery after a crash.