Jean,
I, like many of us here I'm sure, came from a home where you finished what was on your plate. Anything short of that was being wasteful and insensitive to those who didn't have food on their plates. My parents certainly never intended to set me down this path of morbid obesity, but this skill was ingrained in me at a young age. I noticed that I was doing the same with my own children (four of them) and even my wife at times. If they wouldn't finish their food at a restaurant, I would often finish it myself (after eating my own) so as not to offend the server or anyone else who might notice how "wasteful" my kids were being.
It took time for me to come to this realization, but I'm there. I encourage my kids to only eat what they know they'll like (they can experiment from someone else's serving if they'd like) and, if they can't finish, asking for a container in which to take the food home is a good thing. I no longer hover over them making sure they "clean their plates."
I still talk to them about being wasteful and how there are starving people in the world, but the onus is no longer on them to cure the worlds ills.