Internal conflict.  The evil twin of external conflict (conquered in a previous post) and son of the dark lord who shall be named Conflict...also a member of The Big 5 (story essentials).  I described external conflict as a kind of glue that holds the reader.  Internal conflict is glue x 2.  Your character can run, he can hide, but sooner or later his inner conflict is going to find him. 

...AND...

There may not be anything he can do about it.  Maybe the best he can do is barely control it.  Just ask Hulk alter ego Bruce Banner.  How about Harry Potter?  He had his arch enemy's thoughts playing in his mind like an all night radio show.  They tormented him and he couldn't turn them off.  He could only push them away until...well...you know.

Internal conflict is the perfect way to drive back story ("Luke, I am your father").  Talk about your character's inner conflict and you open up a whole world of issues that you can sprinkle in throughout the story.  For some authors, it becomes the second story that they use to mirror the main plot (a perfect example is The Ghostman by Roger Hobbs).  Internal conflict is like an onion that you peel very slowly to keep your readers wondering, worrying, or hoping.  It's all there for the taking when you unleash that evil tormentor called internal conflict.