The answer to this question lies in the knowledge that Nick, the character in this short story, is a damaged and psychologically wounded war veteran who has come into the woods to find some form of healing and peace from his wounds. The nature that is described in this short story is so powerful and has such a massive impact on him precisely because this is what he needs at this stage in his life. He significantly stays away from "the swamp," symbolically representing the quagmire of human emotions and relations that he is unable to cope with at this stage of his life. Note the following quote that explains how happy Nick is in nature:
His muscles ached and the day was hot, but Nick felt happy. He felt he had left everything behind, the need for thinking, the need to write, other needs. It was all back of him.
The landscape has such a big impact on Nick because it allows him to escape human society and gives him space and peace to just focus on bare necessities such as surviving and being by himself. It gives him time to recuperate and to make steps towards wholeness and inner healing.