To have manners is either instilled, appropriated or influenced by traditions. But sometimes it is hard to take the same part in a group like other members with other standards.
To avoid this situation or to exclude other persons and let them feel uncomfortable at the dining table
"Don´t let it to a habit" makes it easier. It helps to hush up the possible differences between all members.
With using „Don´t let it to habit“ there is no chance to see wrong behaviour, because everyone has the same standard.