The ending, or the twenty-first chapter of the book, provides closure to the book for some readers. In fact, this is the only chapter where our protagonist-narrator experiences growth, or more profoundly, personal transformation. In fact, we dare say that given his newfound discontent with violence and violent music, and interest in forging a family, Alex is all grown up.
Structurally, it balances out the other two parts of the book, each with seven chapters. Thematically, it comes full-circle, starting off with the same question and description combination as chapter one in part one of the book, but closing the loop with Alex rejecting the person he was at the commencement of his journey and looking forward to a new kind of life.
That would be the easy interpretation. In the US publication, this twenty-first chapter was left out of all published versions of A Clockwork Orange. What's even more interesting is that Stanley Kubrick's famous film adaptation of the book was modeled after the twenty-chapter version.
So: why is the twenty-first chapter even necessary?
Burgess hints at the answer to this, suggesting that politics or different regional aesthetics had something to do with leaving out the twenty-first chapter. Perhaps it's because the twentieth chapter, with evil prancing all over the page, is sexier. Perhaps the optimism of the twenty-first chapter is at odds with the rest of the work. Or maybe, it's that the twenty-first chapter isn't optimistic at all—perhaps the society that's forcing Alex to grow up and settle down is, in fact, just a more pervasive kind of Ludovico technique?