Walking along the pond, enjoying the animals, Thoreau believes that his solitude makes him a part of nature and therefore allows him to achieve a sense of liberty. When he returns to his house, he can sometimes tell that visitors have been there in his absence. He believes his life is as solitary as if he lived on the prairies or Asia or Africa and that he has "a little world all to [him]self."
What makes solitude worthwhile to Thoreau is the freedom it affords him, being bound to no one and to no institutions, just like nature. Thoreau takes spiritual pleasure in being alone, which makes him feel that he could be anywhere.
From nature, Thoreau gets "the most sweet and tender, the most innocent and encouraging society," which prevents every possible occasion for melancholy. Would anyone consider a dandelion in a pasture to be alone? he asks. He is a part of nature, and no part of nature is ever alone. Once, he says, he did wonder if the company of other people was necessary for a good life, but even then he knew that the thought would pass.
Nature supports Thoreau's isolation from others because it prevents him from ever really being alone. The company of animals, plants, and the elements is an inexhaustible source of spiritual nourishment for him.
Some of his most enjoyable hours, Thoreau writes, were the long rainstorms in which he stayed in his house thinking. Loneliness is a state of mind, he believes, which cannot necessarily be cured by being physically close to someone. Just as people can be blind to the life of nature happening around them, Thoreau believes, they can be bad companions. Moreover, even the best company becomes wearisome after a while. There is never reason to feel alone, because the whole planet is just one infinitesimally small point in space.
Thoreau is making a point to differentiate between solitude and loneliness, which one can feel even when one is in the company of other people. In fact, Thoreau argues, it is solitude, not society, which prevents loneliness. Even in solitude, one is connected to all things.
Thoreau believes that people are distracted by being polite and that they spend too much time around each other, which actually makes them respect each other less. Though he has had very pleasant visits from all kinds of people, he thinks people are more lonely when they are around other people than when they are alone. What prevents the student or the farmer from being alone is not people but their work, which fulfills them.
Thoreau criticizes society for the way it prevents a person from enjoying solitude, which fosters his connection to himself and therefore allows him to create real connections to others. The work he does in solitude enriches him and gives him spiritual purpose.