I've been thinking about external and internal conflict. Sometimes you can be writing along and the story isn't quite working - and then you realize that the reason it's giving you trouble is that it doesn't have both external and internal conflicts, only external.
Why is it so important to have both external and internal conflicts in a story?
One reason is complexity. A story with only external conflicts feels somewhat shallow to me, no matter how thorny the external conflict becomes. I always like to have the sense that the protagonist is somehow at odds with the external realities of his or her situation.
Another is unpredictability. I always like a story that is hard to predict, and as Janice Hardy notes, when you have external conflict driving the story in one direction, and internal conflict driving it in yet another, "crashing them together," the story's direction becomes more difficult to guess.
Both of these are reasons I've thought through before, but here's another thing to consider.
It feels more real.
Now, why would that be? Maybe because it's plausible to think that most people have internal issues they're working through. On the other hand, why would it be so appropriate to have the protagonist dealing with internal conflicts that directly contrast with the external ones? Wouldn't that feel more coincidental? To me it doesn't. After much thought, what I've decided is this:
An internal conflict that contrasts with an external one lets the protagonist fight directly against the author.
As a reader, I know that the external conflict is controlled by the author - an outside force that throws things at our beloved protagonist. I don't feel the same way about conflict inside the character, for some reason, even though I know the author is just as much in control of the conflict inside the character as they are of the conflict outside. I think it's because a well-crafted character will feel like her motivations grow naturally from personality, experience and other factors. Thus, so long as the protagonist feels like a real person, then her struggle with internal conflict, while dealing with the events of the plot, will take on additional dimension. Not only will the protagonist's choices be unexpected, but she will take on that quality that I love to feel when writing a character - the feeling that she is acting on her own against me and against what I might (as the writer) want to make her do.
I urge you to think through internal conflict as well as external. It will really make a difference to your story.