One of the themes of the story is the difficulty of escaping one's past. In moving to the Wyoming valley, Shane hopes to put his previous life as a gunslinger behind him once and for all. He's given up on a life of violence and wants nothing more than to settle down. Yet he's unable to do so, not least because the local farmers are initially rather suspicious of him. Ironically, the only way that Shane can completely gain their trust is through acts of violence in defending them against Fletcher and his thugs. Even then, Shane still refrains from using a gun, relying solely on his fists to send Fletcher's goons packing.

It's only when a professional gunslinger by the name of Wilson arrives in town that Shane realizes he has no choice but to dust off his six-shooter once more. In killing Fletcher and Wilson, there's little doubt that Shane does the right thing; but it's also clear that he hasn't been able to escape his past, and so he must leave the Wyoming valley for pastures new.

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.