It's nice to be gracious about receiving money.
Doug is a homeless man on the street late at night panhandling.
Doug wants other people's money.
When Doug receives a donation he thanks the people and says god bless you.
When Doug receives money from passers-by he just takes it and looks down and doesn't say a word.
More people are likely to give Doug money.
Other people who see Doug act like this are less likely to give him anything.