Old Friend or Boring

I can read some authors' books and feel like it is going back to an old friend while others I get tired of and it just seems like the same old thing.  Wonder what makes the difference.  Andre Norton's books are in the friend category.  I know I have read over a hundred of her books and will even re-read some, which I almost never do.  Where as Sue Grafton's alphabet novels were enjoyable at first but eventually I felt like they were the same old thing and quit reading the last one about half way through.  I just finished one of the Dragon Lance Chronicles Vol 3 Dragons of Spring Dawning.  It was enjoyable and had new adventures even though it was populated with characters I knew.  What is the difference?  Is it me or the authors?