The paradox of giving

I'm always impressed by the vast majority of people who thank me after they've volunteered for a three hour shift of packing food for hungry people at Second Harvest Heartland.  They've often driven a great distance to volunteer, and worked hard repackaging food for their neighbors in need.  In addition to packing, sometimes it involves having to discard rotten potatoes or onions, which will then be given to the pigs.  Other times it might require packaging hundreds of bags of rice or pasta.  Yet, almost to a person, the volunteers are thankful.  Why is this?

I wonder if we don't all feel a  deep-seated need to be helpful and contribute to the common good?  When given a nicely structured opportunity, it is not only welcomed, but also embraced.  Among life's greatest paradoxes is that through giving we receive.