How does Mary Shelley create sympathy for Victor Frankenstein in the prologue?

The prologue to Frankenstein consists of four letters written by Robert Walton to his sister, Mrs. Margaret Saville. The first three letters give a detailed account of his sea voyage, as he traverses the frozen wastes of the Northwest passage. Then, in his fourth letter, he speaks of the "strange accident" that happened to him amid the ice and thick, hanging fog. This accident is, of course, his first encounter with Dr. Frankenstein.
When we are first introduced to Victor Frankenstein, he is in a vulnerable state. Immediately the reader is induced to feel sympathy towards him. As he is being slowly pulled across the ice on his sledge, we see him emaciated, in a state of total exhaustion. Robert is astonished at the politeness Frankenstein shows in asking the destination of his ship. Frozen, weak, and starving as he may be, Dr. Frankenstein still retains a full measure of scientific curiosity. Clearly, this is no ordinary man.
Our sympathy for Frankenstein is heightened further by Robert's description of his features. There is something so incredibly interesting about his face; a certain wildness, perhaps, but also a sweet benevolence in his eyes when someone performs a simple act of kindness towards him. There is also an air of melancholy about him:

For my own part, I begin to love him as a brother, and his constant and deep grief fills me with sympathy and compassion.

Though Frankenstein is made out to be a truly exceptional human being, he nonetheless shares certain character traits with Robert. Both are committed to scientific discovery, yet both are also arch Romantics, with highly-developed appreciation for the beauties of the natural world. But what makes us sympathize with Frankenstein even more is the way in which he appears to transcend the utter wretchedness of his condition, to become a sort of "celestial spirit," a creature almost divine.