One of the moral themes of the story is that love involves sacrifice. Dr. Manette supports Lucie’s marriage to Charles Darnay despite his family history. Sydney Carton not only did not pursue the love of his life, but even switched places with her husband so that they could be together. These are examples of devotion that go beyond the ordinary.
Dr. Manette spent years in prison because of the Marquis St. Evremonde, and the experience left him with a severe case of post-traumatic stress disorder. When he found out that his daughter was about to marry the nephew of the man who sent him to prison, he remained very stoic. He refused to even discuss the matter. He made sure that his daughter knew nothing about it, and timed his mental relapse so that it occurred during their honeymoon. By the time the couple returned, he was more or less back to normal.
It must have been difficult for Dr. Manette to watch his daughter marry the man who was essentially the new Marquis St. Evremonde. He let her do it because she loved him. He also said nothing to her because he did not want to hurt her. He knew that he was fragile, and he made sure that they were not around when he fell apart.
Another example of love and sacrifice is found in Sydney Carton. He was deeply in love with Lucie, but it was a one-sided love. He was well aware that there was no way he could be with her, so he found small ways to be in her life. He just wanted to make her happy.
The biggest sacrifice he made for her was his life. When Darnay went back to France (another example of sacrifice, since he did it to save Gabelle), he was arrested and sentenced to death. Sydney Carton used the coincidence of their similar appearances to trade places with Darnay and go to his death, making the ultimate sacrifice for the woman he loved. His actions are immortalized in the famous last lines of the novel.
"It is a far, far better thing that I do, than I have ever done; it is a far, far better rest that I go to than I have ever known." (Book 3, Ch. 15)