What is the theme in Lucifer's Hammer by Larry Niven?

The will to live is a major theme in the story. The author notes that there are some who fear the comet will bring the end of the world and others who pray it will be so. There are those who are depressed and believe they have nothing for which to live. Maureen Jellison is one of those. She later says that it's one thing to stand on a mountain and consider falling to her death and quite another thing to actually take that step.