Vronsky's life is both contented and simple because he follows his own, unquestionable set of rules, which govern all the possibilities in his narrow range of experience. This includes extramarital relationships: he knows all about how to treat a woman he's sleeping with on the down low (i.e., treat her with all respect; her husband's just an interfering tick; if the husband challenges Vronsky to a duel, he'll be available to answer it).
Lately, however, his relationship with Anna has been taking new directions outside of his comfort zone, and he doesn't know what do anymore. For instance, he doesn't know how to deal with the fact that Anna is pregnant. This is a new situation for him.
He decides that he can't take her away if he continues serving in the military.
This line of thought leads him to think about his guiding passion: his own ambition.
He made a mistake early in his career in trying too hard to display his independence.
For a time, his affair with Anna distracted him from his ambition.
Last week, however, his childhood playmate and colleague of the same age, same wealth, same education and same rank, Serpukhovskoy, was promoted twice and awarded important distinctions unusual for an officer of his youth.
Vronsky decides that it actually doesn't matter, because he has something more important: Anna's love. With her love, he decides, he has put his emotional affairs in order: Anna's like that ten thousand rubles he borrowed from that moneylender in the previous chapter.
He now feels serene and resolute because he has put things in order.