Mottainai (もったいない) is an important Japanese word that helps to explain how Japanese people think. It can be translated “what a waste!”.
Japan is an island nation with few natural resources and a high population density. Historically, Japanese people had to be careful with resources. Japan has experienced many famines in its history.
Mottainai is (essentially) a philosophy: it’s bad to waste anything. For example, it’s considered good karma to finish your plate (not waste food). Mottainai also applies to taking good care of your possessions.