On Characters Changing

The narrator of Lolita has this to say, about the work of constructing selves for our friends:

"I have often noticed that we are inclined to endow our friends with the stability of type that literary characters acquire in the reader's mind. No matter how many times we reopen 'King Lear,' never shall we find the grand king banging his tankard in high revelry, all woes forgotten, at a jolly reunion with all three daughters and their lapdogs. Never will Emma rally, revived by the sympathetic salts in Flaubert's father's timely tears. Whatever evolution this or that popular character has gone through between the book covers, his fate is fixed in our minds, and, similarly, we expect our friends to follow this or that logical and conventional pattern we have fixed for them. Thus X will never compose the immortal music that would clash with the second-rate symphonies he has accustomed us to. Y will never commit murder. Under no circumstances can Z every betray us. We have it all arranged in our minds, and the less often we see a particular person the more satisfying it is to check how obediently he conforms to our notion of him every time we learn of him. Any deviation in the fates we have ordained would strike us as not only anomalous but unethical. We would prefer not to have known at all our neighbor, the retired hot-dog stand operator, if it turns out he has just produced the greatest book of poetry his age has ever seen."

Is it normal to object this much to your friends changing, or is the fact he feels this way just one more sign that Humbert Humbert is a sociopath?

With this paragraph, Nabokov prepares us to encounter a minor character who's completely transformed himself since the death of his wife -- this being, in fact, an example of a time in somebody's life where self-reinvention is likely. The paradigmatic example of such a time is of course adolescence, and Robert Jay Lifton went so far as to hypothesize that "any adult change requires some reviving or perpetuating or recapturing of the tone of adolescence."

Writers of fiction understandably prefer characters who are undergoing changes. One might even propose it as part of the essence of a story that the protagonist is a different person at the end than at the beginning of the story, and that the story takes us through the process of the change.