He thought she would know the mask from the first time they met.
He thought she would run off if she knew it was him.
Those two sentences did not make sense together. Until she put the missing bit of the puzzle in the middle: As soon as he realized that she didn't recognize him, he'd decided to keep that knowledge from her. It really didn't matter why he'd done it, just that he had.
"You lied to me," she accused him. "You lied. How can you say you love me and be willing to do that?"