I would say that one of Pasternak's strongest thematic applications in the novel is how love can play into the collision between the ideal and the real. Zhivago is a poet, one who believes in the idealized notion of love in both the personal and the political. The setting of the novel is one in which the dynamic of Russia is changing into this collision between love in its idealized form and in its most practical base. The ideal is absent in Russia, which, to paraphrase Pasha, has been killed by the revolution. This is where I think that the ideal of love has been challenged by the practicality of love. This changing dynamic is why Yuri struggles so much with love in both his political and personal realms. He is unable to fully grasp how to make practical the experience of love, something that resides in the ideal for the poet. It is for this reason that Yuri withdraws from both Tonia and Lara, the real and ideal, and also why he essentially withdraws from the political reality of Russia. Contrast this with someone like Komarovsky, who has no problem seeing love as something rooted in the practical, and one sees how love is set on a spectrum in which characters must vacillate between the polarities of love as seen in the ideal with Zhivago or in the practical like Komarovsky. In this one, the theme of love in both its ideal and real form challenges all of the characters in one way or another.