Nature is rampant in the narrative of Mary Shelley's novel Frankenstein. It is, literally, everywhere. However, from the many conclusions that we can reach as to the need for it in the story, we can certainly agree that nature serves a protective, nurturing, and curative role in the life of Victor. This may be because after all, Victor has to reject humanity. He violated humanity by trying to act like God. Instead, he creates a monster. Moreover, he has also tampered with nature in such a way that, now, he has to go back to it, perhaps to redeem himself.
Victor has to reject humanity and favor something else to make his life make sense. He cannot count on people to bring him happiness, peace, or company because he knows that everyone around him is in danger of the monster. Victor's search for nature is basically an admittance that what he has done no longer allows him to be considered one with humanity any longer. We could argue that, what Victor does in creating the monster is so abhorrent, that his want to be almost godlike has taken away his humanity altogether. Therefore, in order to keep his sanity he must resort to nature, the world's playground, to try and find himself again.
It is nature, and not the help of his friends or family, that keeps Victor from, literally, going insane. First, we find him looking for solace in nature after his brother, William, is killed by the beast and, by default, the kid's poor governess, Justine,  is acused and executed for the murder, unfairly. This is one of those instances where Victor will start breaking down, and he will look to nature for his cure.
I remained two days at Lausanne, in this painful state of mind. I contemplated the lake: the waters were placid; all around was calm, and the snowy mountains, [...]. By degrees the calm and heavenly scene restored me [...].
Victor's mentality is so challenged by his actions, that not even Elizabeth, or his father, or Henry Clerval, can help him get better. Only nature seems to be able to do the trick
Observe [...] how the clouds which sometimes obscure, and sometimes rise above the dome of Mont Blanc, render this scene of beauty still more interesting. Look also at the innumerable fish that are swimming in the clear waters, where we can distinguish every pebble that lies at the bottom. [...] How happy and serene all nature appears
In another example, we find a similar situation after the monster kills Elizabeth, and Victor is led to near madness. It is nature that he invokes to reach a less insane place in his heart:
What became of me? I know not; I lost sensation, and chains and darkness were the only objects that pressed upon me. Sometimes, indeed, I dreamt that I wandered in flowery meadows and pleasant vales with the friends of my youth.
Therefore, what this leads us to conclude is that Victor has lost his "spot" in the human race by trying to act like a god, giving life to inanimate matter. Moreover, in doing this, he has also tampered and disrespected nature. As a result of his experiment, he loses touch with the rest of the world, as everyone around him is in danger of the monster. Hence, the only thing Victor can really do is find himself in nature, which has been kind to him enough to heal him, and keep him sane. This may be a way for him to make peace with nature one more time, as if trying to redeem himself with it again.
