What is the most plausible reason for a seventeen year-old selling her soul to the devil?
Maybe she is depressed and has a suicidal wish, and at the same time she hates both herself and the world, she wants revenge on something, and the devil (able to tempt her easily in a time when her mind is so vulnerable) promises that she can have revenge (or whatever) if she sells him her soul. It works if the 17yo has a mind which is vulnerable at the time, for any reason (depression: over a divorce between parents, breakup, fight, wants to be popular, the boy of her dreams thinks shes ugly etc / wants revenge for something: stolen, beaten up, raped, etc.) thus making it easier for the devil to trick her into selling her soul.
