Why do the mountains and forest bring peace to Victor but not to his monster/creature?

I would like to add to the previous answer. Frankenstein has always sought solace in nature, because of the natural perfection & Romantic ideals the first response mentioned, but also because he feels most comfortable with himself. We see this throughout the novel, including when he locks himself away in his apartment to devote his time to his creation. Even when he is in the company of Henry Clerval, he prefers to be alone, contemplating human nature and the complexities of life. The solitude of such settings as the forest and the mountains bring him that peace, because he is alone in his own mind. It is only there that he can realize true contentment and essentially become enlightened. All of this connects to the Romantic ideals of nature inspiring invention.

However, the creature has had a completely different life. As the first poster pointed out, the creature is a product of science, & so does not belong in nature. However, there is another level to his discontent. All he has wanted, since his first moment of life, is love and acceptance from humans, and a place in society. But all he's encountered is horror, disgust, and rejection, even from the one who created him. He has been forced to exist in nature, away from human eyes. Because of this, his surroundings continually remind him of what he can't have: a normal human life. Nature only emphasizes his loneliness and isolation. Thus, he can never find the solace in nature that Victor finds. His true contentment would be found in acceptance, family, and love.