I've been reflecting lately about what makes the difference between times when we're full of energy and are able to accomplish big things and other times when even menial tasks seem hard. The starting point was reading a few chapters of this classic book - Flow: The Psychology of Optimal Experience by Mihály Csíkszentmihályi. (chick-sent-mih-high) So far his idea is that People are happiest, most satisfied when they are working hard toward a challenging but attainable goal. It is at this moments when consciousness about self and time fade to the background and you have boundless energy to keep working toward your goals. 

This seems to fit perfectly with your post, the idea that until you can "see" a way to help that is attainable, (helping these 15 people, not all homeless) that we feel invigorated, capable of helping, and that its a pleasure to do so.