When we are serving, I have to keep all of the allergy-free food in a separate place and tell the diners where it is. I still have people, who have been coming here for 40 years, insist on looking over the reserved food. "Weird Uncle, please do not take food from the top of the dishwasher until the Allergy family has been served." Twice a year, for 40 years! And it's almost the same food as the other side of the room! A different kind of turkey ($$) and hard to get, plain potatoes and other vegies instead of buttered, no gravy, just broth from the special turkey. And similar special stuff for the summer gettogether.
Every visit! G-r-r-r-r. I think dementia might be setting in, but that doesn't excuse the 35 years before that.
There is a mindset, in some people, that "the grass is always greener" - in plainer language, the "reserved" food must be BETTER than the rest of the food, somehow, and it is being set aside for OTHER people, not you. Therefore, you deserve a "taste" of it so that you know that you are special and loved, too.
Even if it is kosher for Passover, gluten free, peanut free, pepper free, and flax seed free for a relative with allergies and it is the time for that religious observance. And if Weird Relative is really a pain in the asterisk, they will serve it with a spoon that has been dipped into some other dish that will contaminate it....fortunately, that was NOT a member of my family (and the restricted diet was a lot less strict than the hypothetical example).
Because a Weird Uncle (Aunt or cousin) never ages past four when it comes wanting to get just a "taste" of EVERYTHING in the kitchen - I swear I've hear a story that one person of that ilk was found dipping a spoon into the bacon fat Grandma had on the stove, just to see what it was and get a "taste". (And that container was only "tasted" the one time, according to the storyteller.)