How is the theme of love used in A Tale of Two Cities by Charles Dickens?

Love acts as a driving force in the narrative of A Tale of Two Cities.

It is love that reunites Lucie Manette with a father she has never known. After Dr. Manette has been imprisoned in the Bastille for eighteen years, Lucie Manette learns that her father has been freed. She finds him a broken man who perceives himself as merely a shoemaker, an occupation he has been taught while imprisoned. Now staying in a lodging of his former manservant, Ernest Defarge, Dr. Manette does not recognize his grown daughter, but when he catches sight of her hair, an old memory returns to him. He takes from a blackened rag "not more than one or two long golden hairs," and he marvels as old memories of his wife return to him in the presence of the young woman. Lucie holds her father lovingly and says,

"If you hear in my voice any resemblance to a voice that once was sweet music in your ears, weep for it, weep for it! ...I will be true to you with all my duty and with all my faithful service...." (Book the First, Chapter 6)

Aided by the competent Mr. Lorry, Lucie Manette brings her father to England, where she can care for him. With Mr. Lorry's friendship and his daughter's love, Dr. Manette regains his health in England. Later, he returns this love by permitting her marriage to Charles Darnay, the son of one of the two brothers who caused his arrest and imprisonment, who has fled France after the start of the revolution. 

Darnay pledges to love and cherish Manette's daughter and to protect the physician, as well:

"...dear Dr. Manette, I look only to sharing your fortunes, sharing your life and home, and being faithful to you to the death." (Book the Second, Ch. 16)

Further in the narrative, Darnay finds himself returning to France and risking his own safety in order to defend his agent, Théophile Gabelle, imprisoned by the revolutionaries for his connection to the Marquis St. Evrémonde, Charles's uncle. When he returns to France, Darnay is himself arrested as an Evrémonde, the enemy of the revolution. This arrest brings Dr. Manette to his defense, but he is unsuccessful as Darnay is imprisoned.

It is only the completely unselfish love of Sydney Carton that saves Darnay from the guillotine. Earlier, he has pledged his unselfish love to Lucie, and he keeps his promise:

"For you, and for anyone dear to you, I would do anything.... Think now and then that there is a man who would give his life to keep a life you love beside you!"( Book the Second, Ch. 13 )

Carton makes the ultimate sacrifice as he gives his life so that Lucie can have her husband returned to her by doubling for Darnay, who has been sentenced to the guillotine. Then, having learned that her revenge against the Evrémondes has been foiled, the vengeful Madame Defarge rushes to destroy the wife of Charles Evrémonde. But the fiercely loyal servant Miss Pross defeats her. When Madame Defarge attacks her,

Miss Pross, with the vigorous tenacity of love, always so much stronger than hate, clasped her tight, and even lifted her from the floor in the struggle they had. (Book the Third, Ch. 14)

Miss Pross tells her enemy that she will hold her until one of them faints or dies. In their struggle the gun Madame Defarge holds is fired off, and the hate-filled Therese Defarge falls to the ground.

In A Tale of Two Cities, unselfish love triumphs in the end.