I was raised as a child to eat everything on your plate or you'll end up with an ugly wife, or because of the starving kids in Africa, or just plain wasteful. Whatever reason it might of been, I still practice what was preached to me.
As I grew older and met more people, I realized why my parents and especially my grandmother told me to do so. My assumption; the biggest reason why your parents make you eat everything on your plate - so you do not become a picky eater. Also, so you don't waste food. Thirdly, so you learn to only take what you want.
Eating everything on your plate is not a matter of force-feed. It is strategically used so that a child can get his/her nutrients. "Eat your food and then you can play"; "Can't watch TV unless you eat your vegetables"; "You can't leave the dining table until you're finished"; etc. Now, whether or not it creates obesity, I am sure opinions can vary. I've always been a chubby guy and I always clean everything on my plate. I mean, garbage disposal is an honorable nickname. Had I chose a small plate my entire life, problem solved. But I love to eat too much. I'm not a picky eater, nor am I a selective eater. Some may say "We've already had Mexican food yesterday, let's do something else!". Not me. As long as it is good, it will be in my face. This is why diets have always been so hard for me, and for the past few months, life hasn't been easy.
Anyhow, I'll tell you why my assumption is a borderline certainty - I have never met a person who is not a picky eater but finishes his/her plate (Did I say that right?). I have both the "I'll try anything" friends and the "Eww, what is that?" crew. The "I'll try anything" friends are always up for a good meal, even if it's bad, they eat it all. The "Eww, what is that?" bundle will only eat what they like, and even then they still won't finish their plate because they're "full" ("Full" may be the alternative definition of "I didn't like that food too much"). I live with a person who eat maybe 5 things and I work with another who maybe eat...... 4 things. If you know anyone like them, you can always expect something left in the refrigerator, whether is half of a meal or a sip of a drink. Secondly, expect a to-go box when dining out with this crew, even if it is delicious and the portion is about the size of a kids meal, they won't finish the plate - some do let it go to waste. Growing up, I had a buddy who I prefer to sit next to as you can bet he will have leftovers every time, every meal. I also knew someone who always left the last two bites of every hamburger because she is stuffed. Needless to say, she always had room for dessert. There are a ton more, all of you can relate I'm sure.
On the other hand, I feel less compel when dining with the "I'll eat anything" friends. You go out, the first restaurant that looks good, you sit down and eat. There are no regional/ethnic concerns, no research the menu, etc. You sit, you eat, you leave. The entire transaction is smooth; no bumps, no bruises, and expectedly, no brown bags and Styrofoam containers.
Whether it's a child being fed, or an adult eating out, cultural influence may also play a role in this behavior. It is one big mixed signal. In one country, finishing everything on your plate is a way of paying respect to the server, host, the cook, or even if it's leaving something "for the Gods". It could also be rude and offensive in another, as if the host did serve enough food, or the food wasn't tasty. I guess am grateful for my culture and should thank my parents for the development of my cultivated pallet. Without it, I could never imagine swallowing sea urchin, sea cucumber, cow tongue, tripe, tendons, etc. A clean plate go to with it.