In Chapter 10 of Book II, Darnay declares to Dr. Manette the love that he has long harboured for his daughter, Lucie. Dickens tells us that:
He had loved Lucie Manette from the hour of his danger... But, he had not yet spoken to her on the subject...
Recognising how important Dr. Manette is to Lucie, Darnay decides to speak to him first before speaking to Lucie. Thus Darnay declares his love in rather traditional and stilted language:
"You anticipate what I would say, though you cannot know how earnestly I say it, how earnestly I feel it, without knowing my secret heart, and the hopes and fears and anxieties with which it has long been laden. Dear Doctor Manette, I love your daughter fondly, dearly, disinterestedly, devotedly. If ever there were love in the world, I love her."
One cannot doubt the sincerity of Darnay's serious language, and yet the alliteration in "dearly, disinterestedly, devotedly" seems to undercut the fervency of this revelation, making his declaration somewhat lacking in substance, although obviously very powerful.
In Chapter 11, on the other hand, we have another revelation of love for Lucie, but made in very different terms. Stryver dangles the information in front of Sydney, trying to make him guess who he has decided to marry. When he does not want to play, Stryver tells him, saying:
"I don't care about fortune: she is a charming creature, and I have made up my mind to plese myself: on the whole, I think I can afford to please myself. She will have in me a man already pretty well off, and a rapidly rising man, and a man of some distinction: it is a piece of good fortune for her, but she is worthy of good fortune."
How different a declaration! The manner of playing with this information in front of Sydney indicates a frivolous desire and the declaration when it comes is all about Stryver's needs and wants, and how he can please himself. He uses the declaration as another opportunity to boast about his prospects and assumes that his offer will be accepted because it would be "good fortune for her", not worrying about Lucie's feelings in the matter.
Two very different declarations therefore, but you really should compare these declarations with Sydney's declaration of love for Lucie in Chapter 13. Darnay's declaration is stilted but sincere, Stryver's declaration is inappropriate and arrogant, but true depth of feeling and emotion is revealed by Sydney's declaration in this Chapter.