When we are most profoundly ourselves

Indeed, to the question as to what distinguishes the human being from an animal, as to what is specifically different about human beings, the answer has to be that they are the beings that God made capable of thinking and praying. They are most profoundly themselves when they discover their relation to their Creator. Therefore the image of God also means that human persons are beings of word and of love, beings moving toward Another, oriented to giving themselves to the Other and only truly receiving themselves back in real self-giving.

Joseph Ratzinger (later Pope Benedict XVI),

In the Beginning…: A Catholic Understanding

of the Story of Creation and the Fall

You can see why this book is so deeply satisfying. This excerpt can't give you a sense of the logic that Ratzinger follows but it leads inexorably to the point above. 

As someone who had God nudge them into the St. Vincent de Paul Society, I can testify about "real self-giving." It has been forcibly brought home that when Jesus talks about the judgment of the nations (Matthew 25:31-46), he's pointing out that helping our neighbors is not only for their own good. It is also for our own full flowering as the people he means us to be. At least that is the path upon which I have found myself.