Wendell Berry has the best answer to this question:
“Why do farmers farm, given their economic adversities on top of the many frustrations and difficulties normal to farming? And always the answer is: “Love. They must do it for love.” Farmers farm for the love of farming. They love to watch and nurture the growth of plants. They love to live in the presence of animals. They love to work outdoors. They love the weather, maybe even when it is making them miserable. They love to live where they work and to work where they live. If the scale of their farming is small enough, they like to work in the company of their children and with the help of their children. They love the measure of independence that farm life can still provide. I have an idea that a lot of farmers have gone to a lot of trouble merely to be self-employed to live at least a part of their lives without a boss.” (Bringing It To The Table)
I could not agree more.