Have you ever wondered why it is considered polite to place your napkin in your lap at the dinner table? or, why it is socially unacceptable to stare at someone? have you ever wondered why being late is considered rude in some cultures and not others?

People put napkins on their laps to catch fallen food. 

Well frankly, it's uncomfortable to be steadily gazed upon from a distance.

If you say you will be there at a certain time, you should keep to what you say. It's not kind to keep people waiting in most cultures. Some people don't care though.