Yesterday I got back a novella that my amazing crit partner Kaylee, who was so good as to look over a first draft for me. She is amazingly awesome because she's like a four-year-old -- in a good way -- and asking "Why?" all the time. Sometimes I'm good at internal character dialog and sometimes I'm not. Sometimes I think readers should just get why my character is doing something even though I haven't really hinted at it. Bad idea.
Here's a few other examples of why it's important that your reader know and understand your character's motivation.
What if J.K. Rowling didn't mention that Voldemort killed Harry's parents? Readers start asking, "Why?" Why does Harry care? The fact that Voldemort is evil and Harry is a good guy just doesn't cut it. There needs to be a strong motivation that is clear to the reader about why Harry doggedly pursues his course to defeat Voldemort.
What if Jane Austen left out the part about the Bennett estate being entailed to a distant cousin? The reader would ask, "Why?" Why is it so important for the Bennett girls to marry well? Why is it so important for them to marry at all? Why? Because none of Mrs. Bennnett's five daughters will inherit enough to live on -- and that's clear. She pushes Jane to the point of looking ridiculous because Jane's life almost literally depends upon it.
The moral of the story: In every scene, every chapter, every story arc, try to channel your four-year-old. Keep asking why.