The OP said (in thread title): do you always finish all the food on your plate?
Completely agree it's not a good idea to eat for the sake of it, but surely that means when serving yourself you just put on the plate a sensible helping of all elements of the meal. Taking a big helping and then leaving a lot of it and throwing it away is extremely wasteful, and not a habit I would want to encourage in a child. 'Your eyes were bigger than your stomach' is a useful phrase here.
Obviously if you start eating your meal and it's not nice, you can leave it! (I've had this happen a few times in 40+ years of cooking, always a disappointment for a greedy person like me.)
Equally obviously, if you've misjudged how filling something is, or if you feel unwell, you stop eating.
Eating out is different as somebody else decides on portion size for you.
Takeaways - you decide how much of what's delivered to put on your plate.
Cooking - the quantities I cook are what we need for one meal, and where it's efficient to do so, e.g. casserole, I cook extra to have an extra meal I can freeze or leftovers to eat for another meal in the next day or two. I hate throwing away perfectly edible food.