I've been having a Bonhoeffer inspired season of Advent. This has mostly been hastened by Andy Root's - @RootAndrew - just released book Bonhoeffer as Youth Worker, which I believe (for several reasons) is one of the most important youth ministry books ever written.
Here are a couple of thoughts from Dietrich on Advent.
"The celebration of Advent is possible only to those who are troubled in soul, who know themselves to be poor and imperfect, and who look forward to something greater to come." From Dietrich Bonhoeffer’s Christmas Sermons
"We have become so accustomed to the idea of divine love and of God’s coming at Christmas that we no longer feel the shiver of fear that God’s coming should arouse in us. We are indifferent to the message, taking only the pleasant and agreeable out of it and forgetting the serious aspect, that the God of the world draws near to the people of our little earth and lays claim to us. The coming of God is truly not only glad tidings, but first of all frightening news for everyone who has a conscience. Dietrich Bonhoeffer, Watch for the Light as quoted in Daily Dig, December 18, 2014.