Frankenstein begins the novel as a rather obsessive, determined young man. After the death of his mother, he becomes obsessed with the idea of creating new life. He locks himself away in his laboratory and works determinedly on this project, and in the process, he becomes detached from his family and from the real world. His determination ultimately pays off, at least in the short term, and he creates life out of parts stolen from corpses.
The creature, by the end of the novel, also becomes obsessive and determined. At first, he becomes obsessed with taking his revenge on Frankenstein, who has abandoned him to the cruelties of the world. Then he becomes obsessed with the prospect of a bride, made in the same way as he was made. By the end of the novel, after Frankenstein has destroyed his bride, he is obsessed once more with revenge. He determinedly pursues Frankenstein across mountains and through the arctic, haunting him like a ghost and eventually driving him to his death.
By the end of the novel, both Frankenstein and the creature are lonely and bereft of human companionship, but for different reasons. Frankenstein is lonely because the creature has killed everybody he loves, whereas the creature is lonely because he has been rejected by everybody he has ever tried to love.
Although Frankenstein and the creature are by the end of novel very similar, in terms of how obsessive, determined and lonely each one is, their relationship does fundamentally change. At the beginning of the novel, Frankenstein is powerful and the creature depends upon Frankenstein, as a baby might depend upon its parents. By the end of the novel, however, it is the creature who is powerful and Frankenstein who is dependent. Frankenstein comes to depend upon the creature for his own life, which, ultimately, the creature refuses to give him.
Mary Shelley depicts Frankenstein and the creature as two characters who come to resemble one another to make the point that they are in essence two parts of the same person. The creature is a product and manifestation of Frankenstein's obsession and pride, and the destruction that the creature causes is Shelley's way of showing that obsession and too much pride will always lead to self-destruction. When the creature at the end of the novel drives Frankenstein to his death, what we are really seeing is Frankenstein being driven to death by a part of himself which he could not control. The story of Frankenstein, in this sense, is the story on one man's slow, torturous self-destruction.