What is the theme in Doctor Zhivago by Boris Pasternak?

One major theme in the novel, Doctor Zhivago, is the theme of impossible love. This theme is illustrated through the perfect love Yuri and Lara have for each other..... a love that is impossible, enduring, and based upon short blissful periods that are never marred by the real world. Their love is an escape.... but their real worlds are never far away.