Food etiquitte does not seem to be taught anymore. My children were never allowed to steal from their siblings plates and I certainly didn't let them see me eat their leftovers. However there is nothing more romantic or friendly than sharing a really nice dessert with a friend/lover. A question of good manners or being close with someone?
Its something I have done. If we are out and a bit stuufed from 2 courses but just fancy something sweet to taste at the end, we share. However if I am not stuffed and want my own pud, its hands off!! But I see no harm in sharing something that you both like or having a taste of each others dish etc... its romantic in a way