To grab the last piece of food from the plate to eat together with others without offering to others before it is considered rude. It represents the greedy or not acceptable
Picking up pieces of food on the plate that you shared with any third party without first offering it to others is considered impolite. It represents the excesses or without me.
To pick up the last piece of food dish to eat together with others without offering to others before it is considered impolite. That represents gluttony or uncooperative.