lust - A powerful urge or longing – a deep-seated drive that can sometimes lead to irrational or impulsive behavior, often of a sexual nature.

He liked reading the old story about the prince who had a lust for power that drove him to betray even his closest allies.
reticent - Describing someone who is inclined to keep their thoughts, feelings, or personal matters to themselves, not expressing them freely.

He was reticent about discussing his past, thinking that there were details that she might not react well too.