Sarcasm and Naivete

My recent reading has taken me through the evolution of a few literary characters in a couple of different book series by different authors.
I noticed that with the first series, the Anita Blake Vampire Hunter series, as the story progressed, the books became more jaded and cynical.  It was like a prime example of innocence lost.  Where the first couple of books were exciting and edgy, the latter books became sex-fests with little redeemable value.  I simply had to stop at the halfway point of the twelfth book.  In reality, I should have stopped after the ninth.  Maybe I enjoy embarrassing reading.  I'm not sure.
The series that I'm reading now is the Bailey Ruth Raeburn mystery series by Carolyn Hart.  The first book was very enjoyable, and I was quick to get the second book, which I have already begun to read.  It is a clean series, if not a little innocent and naive.

This brings me to my topic tonight.  Does the evolution of a character have to inevitably fall to jaded cynicism?  I've talked to several people that noticed a significant change in the writing of the Harry Potter series.  After the third book, the characters seem darker and more cynical.  The story is darker.  Is this a function of the story itself or possibly the exemplification of innocence lost?  Is it possible to maintain a primary character's essence throughout a series without our own nature jading the outcome?

Another thought is that perhaps my cynical nature and sarcastic wit would make me unable to write a character with a consistent positive attitude.  Does my view of reality prevent me from allowing such a character to exist?

I hope not, for it is in the imagination and in fiction that our hopes for the world can come to life; and I truly wish for every good situation in all of our walks through life.