(Book 2 Chapters 7-13) Carton surprises Lucie by declaring his love for her. Why doesn’t he want Lucie to return his love?

From the text:

"If it had been possible, Miss Manette, that you could have returned the love of the man you see before yourself--flung away, wasted, drunken, poor creature of misuse as you know him to be--he would have been conscious this day and hour, in spite of his happiness, that he would bring you to misery, bring you to sorrow and repentance, blight you, disgrace you, pull you down with him. I know very well that you can have no tenderness for me; I ask for none; I am even thankful that it cannot be."