Near the end of Doctor Zhivago (1965), after "Strelnikov" dies, Victor again offers to help Yuri and Lara escape Russia. Since the threat to Lara was much more immediate than the first time he made that offer, Yuri and Lara eventually agreed that Lara should leave.
But if Yuri was willing to entrust Victor with the safety of Lara and Lara's existing daughter, and Yuri and Lara's unborn child and the balalaika that was Yuri's only heirloom of his mother, why didn't Yuri go with them?
Throughout the movie Yuri's main goal seems to be to living a normal life with the people he loves in spite of the war, and at this point in the movie Lara's the only one he has left since Tonya, Sasha and Alexander have already fled the country. He didn't appear to be hell bent on tending to the war's wounded or writing banned poetry or anything like that which would require staying behind (that's what he did after choosing to stay, but it sure didn't feel like the reason he stayed). He doesn't seem to be particularly consumed with anger for what Victor did at the beginning of the movie, and has exactly the same reasons to hate him that Lara has. And traveling with Lara wouldn't put her in any more danger than she was in already (as Victor said, Yuri was "small fry" compared to her). So why didn't he take the chance to at least try and start a normal life with Lara outside of Russia where they might all be safe?
I know nothing about the original novel, so perhaps this is better explained there.