The answer to this question can be found in Letter 2 -- the second letter written by Robert Walton to his sister, Margaret.
In this letter, he talks to her about the master of the ship. He says that the man is a very nice man but that he is lonely. He says that the man could have married a "Russian lady." The master had asked the woman's father and gotten his consent, but he found out the woman loved another that her father would not allow her to marry.
To me, this letter is about loneliness. It talks of Walton's own loneliness and about that of the master as well.