What she needs, at least one thing she needs, is companionship. After all why should she eat? Who needs her to be alive? What we call psychosis is sometimes simply realism. But human beings can't live on realism alone.
You can compare Holmes and Watson to great Shakespearean characters, in a way. They've been played by hundreds of actors over the years, and each one is a different interpretation - the source material can take that form of interpretation.