He's just rebuilt his entire body with Harry's blood that has protective love in it. That is what it takes to feel human connection and empathy and grief and remorse; those are the qualities in a person that help you heal. All of the effort that Voldemort has always put into compartmentalizing his soul -- walling off his feelings -- now, those efforts are being countered and undermined by the love in his blood that's a solvent.
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