This is a story that my spiritual director told me back in 1968. 

A very wealthy man adopted a young boy and raised him as his son, with love, discipline, generosity, firmness, vision – everything you could hope for from a father.  And one day the son left, as sons are wont to do, to lead his life in a distant city.  Then one day he realized how blessed he was and how little thanks he had ever given to his father.

He called his father and poured out his heart, and the theme of his words was simple: “How can I ever thank you enough for what you have given me, what you have done for me, what you have made me?”

And the father answered.  “You have just done that, but if you wish to do more, just be the person I hoped you would become.  There is nothing that you can give me beyond that, for I am far wealthier than you can ever realize, but there is a poor man living next to you.  Take care of him as lovingly and as wisely as you can, and if you need anything whatsoever in order to do that, just let me know and it is yours.” 

This is what Jesus is talking about in today's Gospel and explains why Paul reaches the conclusion that he does.