Most novels take chapters to assemble their argument. Anna Karenina delivers its thesis in its first sentence and then spends 800 pages showing what that sentence actually costs a person to live.
"Happy families are all alike; every unhappy family is unhappy in its own way."
It is perhaps the most-quoted first line in world literature. What readers often forget is that the novel is structured around it. Every relationship in Anna Karenina — Anna's collapsing marriage, Levin's anxious courtship of Kitty, Dolly's quiet endurance — is a study of how unhappiness differentiates itself while contentment tends to repeat. Tolstoy is, in a sense, classifying misery.