Usually people tend to find at least a little comfort in being told when they're dreaming, but the TARDIS does have to agree with Johnny's objection, since Zagreus attacked him here. The thought fuels her protectiveness, which she doesn't refrain from sharing with him like a warm heavy blanket.

"I won't leave you," she promises, holding his gaze. "Your dreams are safe now." Not that the scenery and atmosphere seem safe at all yet, still oppressive and stark and labyrinthine. So she brushes away the nasty image gently, instead growing green grass and warm sunshine and a soft breeze, and a million splashes of color solidifying into butterflies. The shame creeping into his emotional tapestry worries her and she doesn't understand it, doesn't see what he has to be ashamed of, but she simply counters it by weaving her affection into the new scene, a sweet scent on the wind.