The famous first sentence of Tolstoy’s great novel Anna Karenina: “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.
This principle can be extended to understanding much else about life besides marriage. We tend to seek easy, single-factor explanations of success. For most things, though, success actually requires avoiding many separate possible causes of failure.
— Guns, Germs, and Steel by Jared Diamond