what is the sympathy that the creature longed for

He doesn't long for sympathy, maybe empathy, but Victor feels nothing but abhorence and regret. What the creature wants most is understanding and companionship;

"The most important feature of this chapter is the way in which the creature convinces Frankenstein to comply with his request. Throughout the better part of their exchange, the creature's tone is reasonable in the extreme: in fact, his desire for a companion seems almost noble. In this way, he will divest himself of his longing for violence and revenge, and lead a blameless life.

By aligning his maliciousness with his misery, he is implicitly blaming Frankenstein for what he has become: such an accusation, however, is effective in evoking the sympathy of both Victor and the reader. The creature often refers to Frankenstein as "you, my creator": this doubled form of address does not only serve to remind Victor of the responsibility he bears for giving the creature life; it is also a complimentary title that implores him for help.

As he speaks, the creature's syntax becomes almost Biblical in tone: he frequently uses the verb "shall," which has the ring of both prophecy and command. He is thus subtly informing Victor that he has no choice in this matter: his acquiescence is already a foregone conclusion."