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Happy families are all alike; every unhappy family is unhappy in its own way.” By that sentence, Tolstoy meant that, in order to be happy, a marriage must succeed in many different respects: sexual attraction, agreement about money, child discipline, religion, in-laws, and other vital issues. Failure in any one of those essential respects can doom a marriage even if it has all the other ingredients needed for happiness.
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Jared Diamond (Guns, Germs, and Steel: The Fates of Human Societies (20th Anniversary Edition))