Everybody is thankful if you help him, why do they need to say "Thank you"...
It's called good manners.
Besides which, have you any idea how depressing it is when you help someone and they just take it for granted and don't even acknowledge that your help has, in fact, been helpful?
And the caveat - everyone likes to be appreciated, especially unpaid volunteers and enthusiasts.
I know that if I help somebody he will say "thank you" in his head or maybe in his prayers.
I'm too old and cynical to believe that, unfortunately - there's a lot of people out there who assume that they have a RIGHT to be helped, however rude or obnoxious they are.
Also just my opinion.