Each time I have asked someone, if they are willing to take a slice from my "yesterday's" pizza, they (most times) immediately become ready to have it. This is very common when you are young, and do not have a lot of bucks to spend on half decent food.
How about, I tell them that there is leftover pizza from yesterday? Again, most times, they will not say "yes". And what if they see that when I show them my leftover pizza and there is only one slice with a bite mark on it? The answer is a big "NO!"
From the above two instances, there are two scenarios with the same leftover food, but the context of leftover is completely different.
This is not only for pizza (which is the most common and popular example for food that is leftover), but true for many other foods, substantially, raw plant-based food.
If people are aware that there is a clear difference between the two kinds of leftover foods, touched and untouched, it will be easier to to be able to share food with one another; especially with the people who are in dire need of it.