I think writing is sort of like acting, except you don't play just one character - you play all of them. Not to mention directing, producing, key-gripping, best-boying... Characters clamor to me, saying, "What's my moti-vay-shun here??" And sometimes I just shrug and reply, "I have no idea." Which is true. I'm making this up as I go.
Because people don't do things just because of their pasts (though that is a factor) or because of their Meyers-Briggs diagnoses (though those are helpful) or because of random events in their lives. All of those things and more combine to contribute to the character at any given moment, and even then they pull surprises.
Sometimes they just respond, and it works. Or sometimes, when I'm falling asleep and thinking about this relationship or that, I realize that someone has held onto an emotion or an opinion about someone or a set of beliefs that are flawed but have been there so long that they've become truth, and that's another missing piece in the story. Just like real life. Inexplicable, but still within the realm of human nature.
And sometimes, when I'm weeding the garden because I'd rather do that than load the splintery old fence into the truck for a dump run and my neighbor asks about my novel and I start babbling away, some little piece falls into place and makes me sigh, "Aha, so that's his motivation." A triangle of blame, so old and buried it seems like a foundation, or like an old, splintery fence surrounding their family.
Aha, even splintery fences can be torn down.