Here are a few passages of Bonhoeffer's book where he explains this in more detail:
"...One who wants more than what Christ has established does not want Christian brotherhood. He is looking for some extraordinary social experience which he has not found elsewhere; he is bringing muddled and impure desires into Christian brotherhood..."
"Innumerable times a whole Christian community has broken down because it had sprung from a wish dream. The serious Christan, set down for the first time in a Christian community, is likely to bring with him a very definite idea of what Christian life together should be and try to realize it. But God's grace speedily shatters such dreams. Just as surely as God desires to lead us to a knowledge of genuine Christian fellowship, so surely must we be overwhelmed by a great disillusionment with others, with Christians in general, and, if we are fortunate, with ourselves.
By sheer grace, God will not permit us to live even for a brief period in a dream world. He does not abandon us to those rapturous experiences and lofty moods that come over like a dream... Only that fellowship which faces such disillusionment, with all its unhappy and ugly aspects, begins to be what it should be in God's sight, begins to grasp in faith the promise that is given to him...A community which cannot bear and cannot survive such a crisis, which insists upon keeping illusion when it should be shattered, permanently loses in that moment the promise of Christian community. Sooner or later it will collapse... He who loves his dream of a community more that the Christian community itself becomes a destroyer of the latter, even though his personal intentions may be ever so honest and earnest and sacrificial."
"Even when sin and misunderstanding burden the communal life, is not the sinning brother still a brother, with whom I, too, stand under the Word of Christ? Will not his sin be a constant occasion for me to give thanks that both of us may live in the forgiving love of God in Jesus Christ? Thus the very hour of disillusionment with my brother becomes incomparably salutary, because it so thoroughly teaches me that neither of us can never live by our own words and deeds, but only by the one Word and Deed which really binds us together -the forgiveness of sins in Jesus Christ. When the morning mist of dreams vanish, then dawns the bright day of Christian fellowship."
"What love is, only Christ tells in His Word. Contrary to all my own opinions and convictions, Jesus Christ will tell me what love toward the brethren really is. Therefore, spiritual love is bound solely to the Word of Jesus Christ. Where Christ bids me to maintain fellowship for the sake of love, I will maintain it. Where his truth enjoins me to dissolve a fellowship for love's sake, there I will dissolve it, despite all the protests of my human love."
Yes, I have many times built ideal dreams of the Christian community in my mind, and yes, the reality is different; but as I have been given the grace to see the imperfections in it and not deny them, I have loved it even more.
For grace to grow, build bridges, and edify lives today,