Some of us grew up in households where you didn't waste food. You might be familiar with ideas like these:
You cleaned off your plate, you were careful to eat everything, because "there are starving children in Africa."
If there was left over food at a group picnic or meal, or after a wedding or a party, you took it home (even if you didn't bring it), because it's shamefully wasteful to just throw away that whole sheet cake (or that box of Doritos, or whatever has been left).
Someone paid hard-earned money for that food, and it borders upon sinfulness to throw it away. If nothing else, you're being a bad steward over the resources under your care.
Perhaps this mindset is appropriate if you're in a situation where you don't eat meals consistently due to financial strain. In those situations, it makes sense that when you have food, you eat it, because you might not get to eat again for a long while. Our bodies are actually designed for just such an occurrence - when we eat more than we can burn off, our bodies store that energy in the form of fat. If we're in a situation where we haven't been getting enough energy from our food, our bodies slow down the metabolism (the rate a which we burn energy) and start using the energy stored in fat.
The reality is that for most Americans, we can reasonably accurately predict when our next meal will be. Most of us can count on getting food as often as we want. Unfortunately, we still think of food as a scarce commodity, so we hoard it - we eat as much as we can - we "stuff" ourselves. I can identify specific experiences in my recent life where I encountered this mind set. Whenever I encounter a food that I really like, my tendency is to eat as much of it as I can, because I don't know when I'll get to eat it again. Take ice cream, for instance. Even though I can go out and buy ice cream as often as I could think to want it, in virtually any flavor I could think of, I eat heaping bowl-fulls, because I might not eat ice cream again for a long time.
My brother offered me some insight into this situation:
It is just as wasteful, if not more so, to pack on pounds, to put our bodies in a state of stress and ill-health, to potentially waste thousands of dollars on medical conditions that could be avoided simply by not eating more than we need. By choosing to eat that sheet cake that you brought home from your cousin's wedding, instead of throwing it in the trash where excessive junk food belongs, you're "waisting" your food, instead of wasting it.
We shouldn't "waist" our food! Throw it out. Junk food is more junk than food. The nutritional value found in it is not worth the calories you add to your system. The choice to have leftover food was made by the people who ordered the food, or by those who prepared it. While you could probably deal with bringing home a piece of cake, unless you have a family of 24 people, a sheet cake is unnecessary.
Leftovers aren't inherently evil. But, if you take home leftovers and eat them in addition to, or in excess, of what you would normally consume, they become a bad idea. Any food eaten in excess of your body's caloric needs becomes "waisted" food.
Save yourself some effort, and make wise use of the foods available to you. Buy less junk and sweets. Make leftovers from meals you cook (take leftovers to work for lunch!) instead of eating it all.
If you are a person who hates to eat leftovers, don't keep them in your fridge OR on your body. Throw them out. Don't stuff yourself just because you can't stand the idea of throwing out "perfectly good food." Food is only good, or useful, to the extent that it helps us to lead healthy, fit lives.
I have to work on this mindset every time I'm faced the the opportunity to "waist" my food. Just recently, I threw out 2 whole pizzas and 6 cupcakes. Why? Because I made the initial mistake of ordering too much pizza, and making too many cupcakes. Even though both were poor initial decisions, there is no need for me to compound it by "waisting" it, too!