It seems that the question bothering Tolstoy for all of his life was: What's the meaning of life? At least, I've seen this same theme in most of his works that I have read, especially the major ones- War and Peace, Anna Karenina, Resurrection and The Death of Ivan Ilyich.
Here, the characters deal with the same question.
We can look at the 3 main guys- Pierre Bezukhov, Nikolay Rostov and Andrey Bolkonsky.
All of them go through a crisis: Pierre- Helene's affair, duel with Dolokhov, break with Helene, meeting with a Freemason; Nikolay- massive loss at cards, followed by deep shame and bad conscience; Andrey- war, a moment he's facing death, the blue sky and then Liza's death. Pierre finds his life idle, selfish, pointless; Nikolay finds himself indulgent, shallow, unprincipled; Andrey finds his desire for glory pointless, a pursuit of false values. Consequently, they have to change.
Here is where the difference lies, both Pierre and Nikolay want to be a better man by living for some ideals- the Freemasonry's principles and virtues in Pierre's case, patriotism and duty in Nikolay's case, whereas Andrey changes in the opposite direction, i.e lives for simpler things, less abstract things, such as himself and the people around him*. His rule might appear selfish and limited, yet a man cannot be a good man if he, whilst trying to live for some high ideals, cannot treat those close to him in a kind way. Andrey is disillusioned twice, 1st with married life, later with the army and its so-called glory.
That is not to say I'm entirely on Andrey's side, disagreeing with Pierre and Nikolay. Nikolay can be a nationalist, with his limitations, but at least he'd doing something for his country, and has a good will, unlike Boris. Pierre can be naive, he's so easily swayed that the moment he meets a Freemason whilst struggling with his bad conscience over all the things he has done, he himself becomes a member of the Freemasonry, but he renounces his past life and tries to be a more virtuous man, to help others. Andrey, at this point, is not only in grief, he's in depression. Haunted by the look on Liza's face after she has died, he feels guilty of having treated her badly. In such a state, he becomes cynical and passive, seemingly abandoning everything. So it's like he's existing, rather than living.
*: My view now, not necessarily Tolstoy's, not necessarily my view in the future.
Update at 12.10pm:
From Volume II part III chapter 1:
"Prince Andrey had been 2 years in the country without a single break. All the innovations introduced by Pierre on his estates without any concrete results, because of his continual flitting from 1 enterprise to another, had been carried through by Prince Andrey privately and without any noticeable effort on his part. He possessed in the highest degree the 1 quality that Pierre totally lacked: the practical application to get things going with no fuss or struggle."The next sentences list his successes.
Thus, without much talking, without big words, Andrey does what he has to do and does a lot more good than Pierre.
The chapter ends with these lines:
"On the journey there he seemed to have reconsidered his entire life and come right back to his 1st conclusion, which was as reassuring as it was devoid of hope- that he needn't bother with anything new, all he had to do was live out his life without doing any harm, free from worry and any kind of desire."