What does 'the saddest of all prisons is a person's own heart' mean?

Nathaniel Hawthorne said "The saddest of all prisons is a man's own heart." ... it is always hard to say what an author meant, because we can't talk to him and actually ask. We have to guess. Sometimes it helps to consider the context in which he wrote... but since this question is out of context, I'll be more general. The idea of being trapped (as in a prison) within your own heart might mean that you have bottled up your emotions so that you can't express them. Being unable to express emotion would probably be damaging to a person, because that would negatively affect interpersonal relationships, and in most cases negate the possibility of a fulfilling intimate relationship with a significant other. Being trapped within your own heart could also have to do with not only emotion, but belief or ideas. Perhaps what you believe and what you want in other ways are in conflict. Remaining in the prison of your heart and not finding a resolution to that conflict could cause a form of self-torture, because going against your beliefs would be painful, and not going against your beliefs would also be painful. Those unresolved conflicts, or bottled emotions would cause a very sad situation for a human being... thus, the quote.