The book explores so many themes, but ultimately, I think the lesson can be summed up by this section which comes on the last page of the book:
What young people didn't know, she thought, lying down beside this man, his hand on her shoulder, her arm; oh, what young people did not know. They did not know that lumpy, aged, and wrinkled bodies were as needy as their own young, firm ones, that love was not to be tossed away carelessly, as if it were a tart on a platter with others that got passed around again. No, if love was available, one chose it, or didn't choose it. And if her platter had been full with the goodness of Henry and she had found it burdensome, had flicked it off crumbs at a time, it was because she had not known what one should know: that day after day was unconsciously squandered.
--[Olive Kitteridge, pg. 270]
It's hard for me to put into words the way this book moved me, but it did. I guess that's the only thing I can really say about it. It moved me.
Well-deserving of the Pulitzer Prize, and the best book I've read all year.