Because motivation is why. It's not just a case of why a character has a goal - which I think comes under backstory - but why is he feeling, doing, saying anything.
In drama, there's a general rule that if you can't remember a line you're missing a "thought bridge". How do you get from one line to the next? The character may be prompted thoughtwise by another character's line, but actually there is almost always a thought bridge from their own last line to this one. It's what Stanislavski calls the "through line of action" and what is normally thought of as subtext. The thought behind the line is the why of saying the line.
Once again, we have the advantage as writers to be able to express the thought bridge to the reader in the character's head. It's not always needed - trust your reader to put it there himself if you've primed the ground. And as a writer you won't need to think this through because it's almost always automatic. But if you stick, this is a good way to unstick yourself.
Look at your character's last set of thoughts. Do they progress logically from what has gone before? Has, for example, our hero been extremely rude so that our heroine has lost it and isn't thinking straight? OK, so now she's calming down and realising what she said back, which provides the why of what she says or does next - apology or defiance, depending on the circumstances.
As a director, you see the play as a series of moments, and direct it moment by moment. Similarly with a novel. It progresses moment to moment - regardless of whether the next moment happens two months later. Each moment has its prompter behind (the why) and in itself provides the next prompt to feeling, thought, speech and action.
If you think with why, you automatically create depth and character because motivation is the meat upon which we all feed and operate in life, and which dictates who we are and who we become.