Interestingly, this blogger informs us that in la belle France, the custom is quite different. She's married to a Frenchman and has lived in France, so she should know a thing or two about the country's customs. In any case, she writes:
If you ever attend a (French) dinner party and plan to take seconds, you must take seconds of all the dishes. For example, if the meal consists of a meat, potatoes and a vegetable, you cannot just take a second serving of the potatoes; you must take a second helping of all three, otherwise you will insult the host.
It makes them feel like you only liked the potatoes so that is all you want to have again.
Or, if they weren't so hyper-sensitive, they could conclude that you simply liked the potatoes the best. Which raises the question: what if you did only like the potatoes? So what?
I love dining in France, but this custom seems more like something that overfed, overweight Americans would have devised.