When I make a dinner, I still have not adjusted to there being only two of us, and eating less for dinner to boot. My joy is to have a second dinner already made to be eaten two nights later. My wife considers that those leftovers are actually lunches, perhaps even many lunches. She seeks wealth in ready lunches, I seek them in ready dinners, and we both try to claim food as wealth for ourselves.
Restaurant leftovers are different. It is their destiny to become lunches, not a follow-on dinner.
Leftover pizza, of course, is breakfast, as is any kind of pie if you are in New England. Well, probably not chocolate or chiffon pies. There are limits. The "pie for breakfast" stereotype referred more to meat pies, but somewhere about fifty years ago some of us started embracing it and fruit pies came to be included as well. I read years ago about a diner that serves a lot of early morning hunters and fishermen (and probably route delivery people as well) that would include whatever was left over from yesterday's pies on its breakfast menu the next day, which sounded like a charming custom to me, but I have never seen it myself. My guess would be interior Maine.