She makes him feel alive again.
When Sherlock died he died along with him. Then Sebastia swept into his life, electrifying him until his heart started beating again. Perhaps that was why it was so easy to let her into his head, despite everything she was and everything she had done. He had made mistakes too. He was a soldier once, wasn't he? And so was she. Another reason why he let her in. He found comfort in finding the reasons why, like it justified the absurd decision.
He told himself he had done the right thing over and over, until he finally believed it. He let go of the apprehension. He let go of the judgment. He held onto her, and he let himself live.
