Ok I concede! Y'all are much nicer than I am.
I eat enough and weigh enough that I don't need to be putting extra food in my mouth on the chance that my hostess will notice my lack of interest in her dessert to the point of having her feelings hurt.
As a hostess, I do sometimes notice what people don't eat, just because I'm cleaning up the dishes. And yeah, as far as I can tell, no one is feeling that obligation to clear their plate. Lots of food ends up in the trash for whatever reason. Maybe they didn't like my food. I dunno. Or maybe their eyes were bigger than their stomach. I don't get my feelings hurt.
But yeah, I get it. Some people might. Food is a touchy subject both for the giver and for the receiver. You cook, you want people to eat. You are a guest, you don't want to make the host feel like you didn't appreciate their hospitality, but you also don't want to be eating food you don't like or can't eat for whatever reason.
I think the fact that dessert is a separate course, served on its own plate, is playing into this. If you go to a potluck and don't eat some of every dish, no one notices or cares. Or if there are multiple offerings at a seated dinner, you might still pick and choose which ones you want. And if you choose the side you brought over the side the hostess made, it might not be as obvious. The number of guests can also make a difference, four at dinner, versus twelve, for example.