To start 2011 off, my first character assessment is for Alaura. She's written in before, and I'm glad to have her again! (It's akin to the good feeling a therapist gets when a client comes back the next week. They must be getting something out of it, right?) :)
Hero left Heroine when their small child died from an illness/accident. She waits a while for him to return, but eventually divorces him. Hero left because of his internal belief that he's a failure. He's a doctor who couldn't save his child. He also believes that leaving the marriage will "set her free," enabling her to move on with her life as if she'd never met him. Since Heroine married him after an unexpected pregnancy, Hero thinks the child was all that bonded her to him.
Alaura wants to know: Besides depression, what psychologically deeper reason would give the Hero his mistaken belief about himself? What could have happened in his childhood/past to make him believe he's responsible for both the child's death and the heroine's anguish following it--and that she'd be better off without him? Also, could there be something deeper and not so noble behind his reason for leaving?
Great questions. You're thinking along the right lines already, so this assessment is really just fleshing out what you've already discovered.
Let's go back to what prompted him to enter the medical field. For many in the helping fields, something happened to them that made them want to be a doctor (or therapist, or social worker). I guess a few doctors go into medicine because they're just good at chemistry, but they are the boring ones. It's the ones who have a reason behind their passion for helping others who we want to read about...the ones with inner demons (which is why House is so popular).
Some examples could be he witnessed the death of someone and didn't know what to do to save the person before help arrived, or he had a loved one die of cancer/disease because of no known cure, or because he himself was saved by a doctor in a life or death situation, and it made such an impression on him that he wants to give back.
Here's an idea. What if he had a brother--maybe a twin, maybe a few years apart--and something happened in childhood where the brother died? What if he saw what that did to his mother and he now sees it happening to his wife, and it stirs up all the feelings from that trauma? The interesting thing is that his mother would have grieved the life of either boy, but because Hero was the one who survived the tragedy, he doesn't realize that both of their lives were equally precious to the mother.
Johnny Cash scenario or Faramir telling his dad that he knew his dad wanted him to have died, not Boromir. As restitution then, to his mother, to himself, to his lost brother, he becomes a doctor, intent on saving lives like the one he lost when his brother died by [insert senseless death here].
Now, in present day, have his child die in some fashion that the Hero would feel it is his fault. Either he was driving the car, or he turned his back for a split second at the pool, or lost grasp of his son's hand at the festival...something physically preventable, but only by a man with inhuman strength. Now make that reason he didn't prevent it have something to do with his internal schema or not being good enough or being a failure. He's been living his life trying to live up to his own ideal of inhuman strength (whatever that looks like for him) and when he didn't have it--didn't come up to par--he's completely dejected.
As for a not-so-noble reason for him to leave, I'd bring it back to that major incident when he was a child. Take the mother thing again. He saw the way his mother grieved, and how dealing with her grief turned his father inside-out. Maybe his reason for leaving Heroine is a bit selfish: he's already got enough to deal with, he just can't handle Heroine's grief trapping him, sucking what little life he does have right out of him. Just a thought. If his dad changed, withdrew, stopped caring about things--maybe became an alcoholic--that would be a huge fear for Hero. He wanted to change the world as a doctor, save people...not ruin himself via the grief of someone else rubbing off on him. Does that make sense?
Lots of couples who lose a child break up over the traumatic period following the child's death. That's a very realistic end, but it's also stereotypical as a result. So the difference in your story will have to all be in the backstories, so good luck with it!