I got the impression that he became disillusioned from his statement that the new government is trying to remake a man (or life) by force when this process goes against the nature of the man. I don't remember the quote correctly, but it went along these lines. From this statement and from various episodes when Zhivago's way of thinking collided with the "correct" way of thinking I got that feeling about his disillusionment.

There was also the episode from shortly before his death when he inwardly chides his friends for not being able to think for themselves beyond what was considered correct.