It's rude not to share with others.
John is at a restaurant with his friends, their food has just arrived and John notices that his plate of fries is humongous.
John wants to eat some fries.
After putting some fries on his plate, John moves it into the middle of the table and invites his friends to help themselves.
John eats all the fries on his plate.
John is offered some food from his friends' plates in return.
John feels really gross and has a stomach ache all evening.