How is David's work life a penance for not being able to save Elizabeth?

David says that he'd initially wanted to be a surgeon because a surgeon has the power to "wow" people. He also relates the story of going with Elizabeth when she'd worked for an organization seeking to get homeless children off the streets. David says that he'd winced at the condition of these young street children but that Elizabeth never had. Now, years later, David says that he admired that about Elizabeth and it could be this that prompts David's decision to work with the poor rather than seeking a lucrative career as a private physician. David says that he is a better person now than he had been years earlier, prior to Elizabeth's death. He admits that some people would point simply at the fact that he's aged physically, meaning he's also aged emotionally, as the reason for the improvements. David disagrees. He says that having Elizabeth by his side meant that he could lean on her goodness, in a sense. He says that now, without Elizabeth, he has to be a better person.