Why is probably the most important word when I’m writing. It’s fab word. It helps you stay curious. As a writer I need to remain curious. If I wasn’t curious about my characters and their motivation then the story wouldn’t move far. It would stagnate once the initial burst of joy and energy vanished. I know plenty of people who have that happen. And they promptly start another story with a hiss and a roar … and it happens again. And then they’re asking why they lose interest and motivation. 🙂
When I’m teaching crime writing, one of the things I say to students is, “Know your why.” Why are telling this story? Why does this matter? Why do you want to write? Why carve out time to write in an already busy life? Why these characters?
Why? Why? WHY?

Little kids are great at asking why. There is a phase when they’re about 3 when every answer you give gets a why response. It can be pretty annoying BUT they’re learning to question everything, that’s how they figure their world out.
They need space to ask why.
Authors need space to ask why. Characters need space to evolve.
If you’re writing and your main character will not behave instead of shoe horning them into your version of the story ask why?
Why did that happen? Explore the why. Follow the new path. NOTHING is set in stone. It hurts no one to run with things and see what happens.
What’s the worst case scenario there?
You have a scene or chapter that takes a different road, could be better, could be worse. It’s not wasted. If you don’t like it, drop it into the outtake file and carry on. (Yeah, don’t delete it, sometimes what you think is all wrong works later in the story.)

What is your why?
What is your character’s why?
Sometimes when a new character starts throwing their weight around I pick up a pen and my notebook and let them go for it.