One theme of "Shane" is that the story is a coming of age story. It's a common theme in a lot of books. "Harry Potter" for example also works with this theme. The narrator of the book is Bob Starrett. He is a boy, not a man. Because the story is told from his perspective, the reader gets to see how his thoughts and feelings change as he gets older, gains experience, and learns from two very influential men (his father and Shane).
A second theme present in the book is the theme of what it means to be a man. "Shane" has a refreshing viewpoint on what it means to be a man. When a person looks at modern day movies, books, and video games, being "a man" is consistently along the same lines. Have muscles, use violence, treat women poorly, be aggressive, be cocky, be an emotional void, etc. "Shane," on the hand, promotes a completely different view of what it means to be a man. A man is someone who doesn't first resort to violence. A man is someone who keeps his word, is loyal, is trustworthy, and trusts others. A man is also someone that treats women in a way that resembles chivalry.