PZ Myers has some good thoughts over at his blog, Pharyngula.
I'm reminded, as I often am, of "A Pail of Air" by Fritz Leiber:
"So I asked myself then," he said, "what's the use of going on? What's the use of dragging it out for a few years? Why prolong a doomed existence of hard work and cold and loneliness? The human race is done. The Earth is done. Why not give up, I asked myself—and all of a sudden I got the answer."
"Life's always been a business of working hard and fighting the cold," Pa was saying. "The earth's always been a lonely place, millions of miles from the next planet. And no matter how long the human race might have lived, the end would have come some night. Those things don't matter. What matters is that life is good. It has a lovely texture, like some rich cloth or fur, or the petals of flowers—you've seen pictures of those, but I can't describe how they feel—or the fire's glow. It makes everything else worth while. And that's as true for the last man as the first."
"So right then and there," Pa went on... "right then and there I told myself that I was going on as if we had all eternity ahead of us. I'd have children and teach them all I could. I'd get them to read books. I'd plan for the future, try to enlarge and seal the Nest. I'd do what I could to keep everything beautiful and growing. I'd keep alive my feeling of wonder even at the cold and the dark and the distant stars."
The complete text of the story may be found here.