Frankenstein begins with Walton writing letters to his sister, Margaret. His first two letters describe his plan to travel to the North Pole to investigate the magnet. They also detail how he is lonely and wishes that he had a friend. While he's excited at the opportunity for adventure and to learn something new, he knows if he succeeds, or fails, no one will be there to celebrate or commiserate with him.
But I have one want which I have never yet been able to satisfy; and the absence of the object of which I now feel as a most severe evil. I have no friend, Margaret: when I am glowing with the enthusiasm of success, there will be none to participate my joy; if I am assailed by disappointment, no one will endeavour to sustain me in dejection.
In his fourth letter, Walton informs his sister that his crew found a stranger. The man was found on the ice nearly frozen to death. Walton's tone has changed. While he does worry about the stranger, he's excited that he may have found his equal: a friend.
I said in one of my letters, my dear Margaret, that I should find no friend on the wide ocean; yet I have found a man who, before his spirit had been broken by misery, I should have been happy to have possessed as the brother of my heart.
As the stranger gains strength, his begins to tell Walton his story of what has brought him to the ship. The stranger hopes that by telling Walton his story, he will prevent him from making the same mistake.
“Unhappy man! Do you share my madness? Have you drunk also of the intoxicating draught? Hear me—let me reveal my tale, and you will dash the cup from your lips!”