If memory serves, he has an interview or a foreword to a book in which he mentions another reason: he didn't want Calvin and Hobbes to continue for years or dec

If memory serves, he has an interview or a foreword to a book in which he mentions another reason: he didn't want Calvin and Hobbes to continue for years or decades after its natural life. He felt that he had seen this with other strips: they kept going long after they should have stopped, and the evidence was that they were consistently uninteresting. He didn't want Calvin and Hobbes to be like that.