The affair between Dmitri and Anna began because they were both bored and because they were both unhappily married. Dmitri didn't really understand his feelings until he got back to Moscow and resumed his normal routine. One night he tried to describe his affair to a friend, but the other man couldn't understand him. As he was getting into his sledge, the friend said, "You were right this evening: the sturgeon was a bit too strong."
These words, so ordinary, for some reason moved Gurov to indignation, and struck him as degrading and useless. What strange manners, what people! What senseless nights, what uninteresting, uneventful days! The rage for card playing, the gluttony, the drunkenness, the continual talk always about the same thing. Useless pursuits and conversations always about the same things absorb the better part of one's time, the better part of one's strength, and in the end there is left a life grovelling and curtailed, worthless and trivial, and there is no escaping or getting away from it—just as though one were in a madhouse or a prison.
This insight motivates Gurov to seek out Anna again. But they have both made irrevocable choices in the past and cannot break free from the lives that have evolved as a result of their previous choices. The best they can do is to see each other for a few days every two or three months. Would they have been better off if they had never met? Would it have been easier for them to put up with their "worthless and trivial" routines? Characteristically, Chekhov does not provide an answer.
Many readers can identify with Gurov and Anna because they too feel they are stuck with "a life grovelling and curtailed, worthless and trivial, and there is no escaping or getting away from it--just as though one were in a madhouse or a prison."
The moral of the story is a bit different than the typical type of moral. The moral of true love is present, but it must be attained by the couple leaving their respective spouses to be together.
The other relationships must end because they are not about love anymore, and the affair must grow into a solid relationship.