I believe that Montag is not in harmony with his society because the society is depriving him of things that he thinks are important. Montag wants, more than anything it seems, to have human relationships. But the society does not encourage such things.
We can see that Montag's life changes after he meets Clarisse. She shows him that people can and should care about one another. When Montag then thinks about his relationship with Millie, he sees that the two of them do not really care about each other. This bothers him, especially when she almost kills herself.
So Montag is looking for more meaningful relationships. Society, however, does not want people to care for each other. It wants them to indulge in mindless and emotionless entertainment. It wants them to end up like Millie and her friends. Because of this, Montag is not in harmony with the society.