Why can't children just keep something they don't want to eat on their plate?

I have two kids, 3 and 5 year olds. Every time they don't like something that is on their play, my Wife and I just say "Just leave it on your plate if you don't like it" (we aren't the type to force or kids to clear their plate or eat foods they hate), yet they insist on putting it on our plates getting upset if they can't. They cannot just leave things on their plate if they don't like it, I don't understand this?​My neices and nephews do the same thing. Do they think that it'll ruin their other food if they don't take it off that instant? via /r/Parenting http://bit.ly/2HK5pdw