Characters have minds of their own.
No, seriously. You think I mean in some philosophical or metaphorical way but I really mean it when I say characters do what they want, not what you tell them to do. You simply write their story down but they choose what they want to do. That is of course, if you have made them well enough.
For example, as I was writing my first novel, the main character changed. And not by my design. Originally, the novel was the story of a man; an American GI at the tail end of WWII in Europe. But as writing progressed, the French woman he falls in love with ended up stepping into the light and making herself into the central character of the novel. She made it HER story. Again, this was not intentional on my part. I fully believe that a good character does what they want to do, whether you intend it or not.
My advice? Let them do it. Don’t be afraid to let your characters make their own choices, even if its not what you expected.