Another reason not to waste leftovers: They often taste better the next day. Chemical reactions in the food continue to take place and produce more or new flavor molecules, according to the Institute of Food Technologists.
Proteins may continue to break down and release amino acids such as glutamate that make food taste more savory (umami); other amino acids react with sugar to produce new flavor molecules if browning occurs during the reheating of leftovers. On the other hand, leftovers may taste blander if eaten cold because our taste receptors are less responsive to foods that are below 60° F; lower temperatures also reduce the volatility of aroma compounds.
Make sure to handle leftovers safely, especially by refrigerating or freezing them as soon as possible.
Also see Where to Keep Cooked Rice.