Like a few others here, I cook for one. There's almost always leftovers.
Some are accidental leftovers, but many are planned. I grill often and there's no way I'm going to grill, for example, one or two chicken thighs for one meal.
Even when I'm grilling a big thick rib eye, I usually throw some veggies on the grill to make the best use of the coals.
About the only things I throw out if I don't eat it all in one meal are veggie salads, fruit salads, and fish. I don't expect fish to reheat well, but I've been known to nibble on a piece of leftover fish the next morning if I especially liked it the night before.
edited...I wouldn't call a cake or pie 'leftovers' though...in my mind leftovers are from a meal and not a dessert. Just like I wouldn't consider a carton of ice cream from the freezer leftovers if a scoop had been taken out of it.