The More Things Change....
Are we who we are because we were born that way, or did our environment shape us? That's the question of nature vs. nurture. We won't settle it here, but in writing it's important to find a story that lets your characters change in significant ways.
Because that's one reason we read stories: to learn how others act and react and what events shape and alter their lives. For that reason, find important stories to tell. Your characters should have something at stake, something to protect, and possibly something to lose - and the process should have an impact on the way they think and act.
It gets to be a problem with series books, of course. Unless you think of the entire series as revealing a very gradual process of growth and change, of loss and gain. Then it works like a charm.
Would A Christmas Carol be a classic if Scrooge had not changed? Don't think so. Would Pride and Prejudice be a classic if Mr. Darcy refused to unbend and Elizabeth refused to reconsider her initial judgments? Probably not. The best style in the world won't make up for characters who just refuse to change in any way.
So...ask yourself "How is my character wrong in his/her thinking and judgment and opinions? How will the character learn better?"
Pursue that. Readers like characters who are open to change. And the more difficult the process is, the more the readers like them.