Having spent many years in the retail trade greeting customers and endeavouring to be helpful, I know the importance of a simple thing- that is a smile!  Have you ever noticed how on a grey day when neverything is getting you down someone comes in with a smile and laughter and you feel better Yes better!  The lesson is a smile a day keeps the frowns away!  Try it make some one smile back at you and nothing will seem quite as bad.
This is the same as curtersy why do we think it is unusual when it should be automatic .