While Sam Gribley does originally plan to run away from home, his father discovers his plans and tells him that he will be allowed to travel to the abandoned Gribley farm to attempt to live off of the land provided that he let people in town know where he is. In this way, he does not "run away" in the traditional sense but is allowed to go by his father, who believes that after a very short time without modern comforts, Sam will return.

This turns out to not be the case at all. Sam leaves his home because he finds his cramped living situation completely unbearable and craves the freedom and adventure provided by living in open nature and off the land. Sam carefully studies survival skills and proves to be extraordinarily talented in regard to being self-sufficient. With his accumulated skills, Sam proves that he is able to live in the wild indefinitely.

In the first two chapters, you’ll find three reasons why Sam Gribley left his home in New York City to run away to the Catskill Mountains. Near the end of Chapter One, titled “In Which I Hole Up in a Snowstorm,” Sam shares some details about his family life. He has four sisters and four brothers; so together with his parents, they total eleven people living in one apartment. It must have been a tight and uncomfortable space.

And not one of us liked it, except perhaps little Nina, who was too young to know. Dad didn’t like it even a little bit. He had been a sailor once, but when I was born, he gave up the sea and worked on the docks in New York.

Sam evidently wanted to get out, to a place where he could be on his own and could breathe. Then we learn that Sam’s ancestors once owned property in the Catskills. His father told him:

“That land is still in the family’s name. Somewhere in the Catskills is an old beech with the name Gribley carved on it. It marks the northern boundary of Gribley’s folly – the land is no place for a Gribley.”

Sam lets us know that he has since found the beech tree and the carved name, and that he thinks the area is just the right place for a Gribley to live.

In the second chapter, titled “In Which I Get Started on This Venture,” Sam tells us that his father once ran away from home but chickened out and was back before nightfall. Sam wanted to do likewise, but instead go off to the famous Gribley land.

Everybody laughed at me. Even Dad. … He had roared with laughter and told me about the time he had run away from home. … Then he told me, “Sure, go try it. Every boy should try it.”

So Sam did. And he found success living on his own, on or near the Gribley land. And part of him wanted to prove especially to his father that he could do it.