When I cooked more often, daily really, I always used leftovers. There are SO many things you can make with leftovers. I even used to save dessert crumbs and heels of bread and make amazing bread puddings (I'm off carbs now, so I haven't baked in months)
I had a friend that hated leftovers. We were there once for a cookout and there was a pile of steaktips and potatoes leftover and she was about to tip them in the bin and I stopped her. Expensive steak in the garbage?! She said, "I don't do leftovers" Well, I took those home and @winstonalden and I ate off those for two meals. Silly really.
In many ways cooking, for me at least, was like artwork and gardening, a bit here some scraps there and tadah, a meal!