Seth appreciates the comforting touch, so it takes him a moment to realise the 'sorry' is an apology, not just an offer of sympathy. They've been talking a while now, so his reason for not wanting to be touched has taken a backseat. He's even a little sad when Daniel stops.

"S'alright," he reassures with a small but sincere smile, not sure exactly how to communicate his openness to touch.