Because it's such a difficult subject to get right, let's talk more about inserting Heart into your novel. Not the Heart of the story, but the Heart of the character, so that the reader cares whether the character wins or loses.
First, what ISN'T Heart... When a character "owes" their life/career/marriage/etc to someone else, that debt does not constitute Heart. Neither does the fear of losing life/career/marriage/etc. These just puts strings on her behavior and represent Motivational aspects in the character's development, also known as their Arc.
What is Heart? Think about what your character would sacrifice whether he/she felt like she owed or not and regardless of his/her fears. That's where Heart is: beyond the self, beyond paying debts owed, beyond fear.
RISE ABOVE THE STANDARD
Heart is about rising above the self for a greater good, a greater belief that is so core to the character it makes them "worthy" of our trust, admiration, and esteem.
Heart is based on one's values/ethics; what the character will die for to uphold. A villain will die to uphold his grip on power, perhaps because it's what makes a man righteous with God in his mind. Icky, yes, but deeper POV than just "he likes power" right? That's Heart, even though it's dark.
Similarly, a hero will fight to save the damsel not because he's meeting the definition of heroic, but because he's driven by fears, hopes, and values. Again, the fears and hopes part is related to his character development. The values part is connected to his Heart--what he values makes him valuable in this world (i.e. worthy of our caring about him in your book) and is the strongest motivation for taking up the mantle of Hero and saving the day.
Like a fireman who will run into a fire to save a child not JUST because his father abandoned him as a kid, not JUST because he's afraid of never feeling loved/worthy in life--lots of folks feel that way and don't run into fires. But he will actually go into that fire, will risk his own life, because of his core beliefs. For example, he might believe a child has no power to defend itself, so it's an adult’s duty to protect the innocent because that's how a sane community (read "family") treats its own. That value would define a lot of his behavior, and make you trust that man with your child any day!
HEART CULMINATES FEAR AND HOPE
So you see, the fears and hopes and pain of the past INFORM a character's core values. In the example above, he is choosing to be the hero he needed as a child to 'right' the world that wronged him. He doesn't want another child to hurt like he did. But even more than this avoidance of pain, he wants to do it because it's right for the higher good.
So yes, Heart stems from fear and pain and old debts, but it's something higher, an elevated belief. It's taking all that pain and fear and incorporating courage and power and a little beautiful humanity.
This is deep psychology folks. Not easy stuff. If you've ever read "A Prayer For Owen Meany", you know that Heart drives the whole book. And it's unforgettable. That's what I want to help you achieve with your story. And once you really understand how to reveal Heart for your character, your future books will get easier to write.
The Editor Devil.