What is the theme in Doctor Zhivago by Boris Pasternak?

Impossible love is a theme in the story. The love that Yuri has for Tonia and that Lara has for Pasha has the sense of normal human relations. This kind of love grows with time and is learned along the way. It can start with admiration and fondness, or even mild attraction. However, the love that Yuri and Lara have cannot survive as a normal human relationship. The civil war tears it apart, but even in peacetime the passions involved may not be tolerable.