Why? I've got a running theory that this is the single most important word a writer has in their vocabulary.
I've been told that I'm a pretty tight plotter. I think it's because I'm like a five-year-old pestering Daddy with questions. Why does an octopus have eight arms? Why do sparrows have two three toes in front while a parrot only has two? Why, why, why? I've always had an insatiable curiosity about things so I've never been afraid of the word why.
I also had parents who weren't afraid to attempt an answer, no matter what. They never threw a "Just because, so stop asking," at me. I'm very grateful for that.
Now when I look at my stories, I've got an endless stream of why running through my head, punctuated by the occasional what.
And I have to have answers to these questions. I can't leave it alone until I do. Maybe that's why I'm so attracted to stories with intricate world building. Good world building shows off the power of why. But why can be a very useful tool when plotting.
Take Hitchcock's The Birds. This movie drove me nuts. It didn't give me nearly enough answers to the whys. Why did the birds attack? Why that town? Why all the birds? Why in the heck did she open the door at the top of the stairs when she heard a noise in there and she was safe with the door shut? Please, someone help me with that one.
Now the horror genre likes to leave out the whys. It gives us a sense of unease when we don't have answers, so I can defer to Hitchcock on that one. However, in Romance, I've got to have those whys. Why is she trying to escape her former life? Why is the hero so attractive to her? Why does he drive her nuts? Why did she open the door at the top of the stairs when she heard a noise and she was safe with the door shut?
This is where why becomes such a powerful tool. If you use why correctly, you avoid the notorious TSTL (Too Stupid to Live) Heroine. You avoid the dastardly for dastardliness's sake villain, and you can avoid a lot of plot holes.
And so I embrace the power of why, and don't ever let myself get away with "Because. That's why." It makes world building and plotting more fun.