The main theme of Thoreau's Walden is very simple. It is that life can be easy and pleasant if your wants are simple. We create many of our own problems by wanting things we don't really need. He thought his neighbors were foolish to complicate their lives and create so much stress for themselves. In the first chapter of his book Thoreau offers many examples of the ways in which his neighbors make themselves exhausted and careworn so needlessly.
I sometimes wonder that we can be so frivolous, I may almost say, as to attend to the gross but somewhat foreign form of servitude called Negro Slavery, there are so many keen and subtle masters that enslave both North and South. It is hard to have a Southern overseer; it is worse to have a Northern one; but worst of all when you are the slave-driver of yourself.
Most of Thoreau's book is thus about how to live simply and the benefits to be derived from doing so. He describes the pleasures of communing with nature, which costs nothing. A lot of his text is about his enjoyment of everything he did, including the small amount of work he had to do in order to raise most of his own food. One chapter is devoted to the pleasure of reading, another pastime which costs little or nothing. Of course he spent a great deal of his time in writing as well.
One thing Thoreau does not talk about is marriage. It is really fairly easy for a single man to live cheaply, even today. But most people want to get married and end up doing so, which usually results in having children. We don't know how Thoreau could have coped with family life. It would appear that he never married because he never found a woman who would share such an austere life as he considered ideal. It is the ideal life for a philosopher or a religious mystic, but not a life that every man or woman could follow. Reading Walden today is somewhat like reading Robinson Crusoe. It is temporary escapism from reality.
The main themes inherent in this work are related to a quest for a good and useful life. Thoreau believed that a close relationship with nature was necessary to human existence; this was reflected in his frequent insistence that living simply was also of paramount importance. Being removed from the life of the city, from unnecessary social events, allowed him to reflect upon what was truly important and to create a coherent and reasonable philosophy of living. Thoreau believed solitude and simple every day rituals were the essence of a fulfilling life and Walden exemplifies these two ideas as well.