This scene in its entirety is one of my absolute favorites in all eight seasons, but in this clip specifically I want to draw attention to:
The change in House's voice before and after Thirteen tells him, "No, you don't know because I don't know." He is so utterly gentle and quiet when he says to her, "How could you not get tested?" There's no judgement there, no anger, no scolding. To me it's pure shock, concern, empathy. He cares. So much.
Thirteen's line: "Not knowing makes me do things I think I'm scared to do. Take flying lessons, climb Kilimanjaro, work– for you." She was scared to work for House. The whole season until then... she's been afraid that entire time. And, save for the moment with the patient in Mirror, Mirror– which we (the audience) are privy to and House is not– she hides it so well.