What does he mean when he says that I make him vulnerable?

The other day he told me that I make him vulnerable and I'm speacial because no one could ever take his "mask off" as easily as I did. He wants to be known as a heartless guy with no emotions but he can't be that with me. He feels like a human again when we're together. He says he likes what I do to him but it also scares him a little bit. He said that no one ever had that kind of power over him
What does he mean when he says that I make him vulnerable?
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