Nikolai Levin

First, let me say that having to read Anna Karenina for literature class has been one of the most rewarding experiences I've had. This is a book that is actually shaping the way I think about life. Anyways, I have to give a lecture on it in class, and I was having trouble with understanding Nikolai's story? Was Tolstoy just putting to paper his mourning over his brother, or does this fit into the underlying philosophy of the novel? If the latter, what is Tolstoy saying? What was "wrong" with Nikolai, which lead to his poverty-stricken life and his death?