In childhood, it's easy for books to become friends (for those who like reading, anyway). Some parents object when their children want the same books over and over, but it's not so different from enjoying the same music album several times, or hanging out with the same friends.
It's probably the over and over aspect that leads some books to become part of us. There are books I haven't read in forty years, but I still remember chunks of dialogue. Even if we never reread these childhood friends they are still part of our make-up.
The friend aspect doesn't go away for adult readers, but I think it becomes more difficult for books to win a place as part of us. Maybe this is because we tend not to re-read as much as when we were children, or maybe we become more critical.
I think over the past decade I've found only one series that has become a real friend (leaving aside books I've written, which are part of me anyway)...with one more that might qualify, but which needs a while to settle into my psyche first.