Progression of Life
We hear Cassiel and Damiel speak of how war progresses through the city of Berlin, and also how the grass and water continue to persist as well. The story of the city is the story of life itself--there is loss of life and heartbreak and there is also the unrelenting nature of creation that continues to grow and bring new life over time. And though man chooses war, it cannot out-power the determination of life to spring through created things.
Desire
Desire is a major, indeed unavoidable theme of this film. We see it in Damiel who desires to be human, to know Marion deeply and intimately. His desire to experience the world sensorily outweighs his position as an angel in eternity. He is willing to trade his eternity in order to be with humanity and experience life in every decision from the smells and sights, to touch and most important to him, love.
As One Thinketh
In the black and white portion of the film, we are with the angels as they go through Berlin to be with the people who inhabit the city. We hear the thoughts of every person they encounter. We can understand that the thoughts of a person dictate their emotional state, and their belief in what can be. Thus, it dictates their actions and these actions, which are decisions will dictate their lives: the quality of their existence on the earth.