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She would always heal him no matter how deep his wounds were. Under any circumstances, she would protect him.
Even if she had been dead for one hundred years, she would still help him.
When she had fallen at the hand of the phantom beast, she had realized she wouldn't be able to help him if she died. While the thought of passing on saddened her in many ways, to not be able to heal and protect him again tormented her the most.
For a century she had been mourning about Hyrule's loss, her own loss, and his loss. Her world was shrouded in darkness, the light that had once illuminated it nowhere to be found. It was when that light came, after a hundred years of gloom and despair, that she smiled. To see him again, alive in flesh and in soul, made her shed a tear in happiness.
It was after her spirit had been freed that she realized she could still help him. While she couldn't heal or protect him, she could give him her power instead. In that way, when he was to fall from the direst of injuries, her power would be able to save him.
Why would she go through all of this just for him? From bearing a burden for a century to lending him her power, why did she even care? For some, the reason behind her actions might be unfathomable. For her, however, the reason was simple.
She loved him.
Always.