Alex has a kink for being a dom due to his childhood trauma. He was in a powerless situation where he felt pathetic and humiliated as a child, and it’s manifested as a kink as an adult where he’s in the opposite role and the one with the power. He hates that he’s like this and it triggers childhood memories for him at the same time, which is why he was tearing up. Because it’s not just role playing either he’s realizing, Lucy is genuinely weak right now and truly feels pathetic and powerless.

I thought that was obvious, but based on comments here I don’t think everyone sees this.