Gon is concerned for Killua because Killua never talks about himself, and always hides things from Gon, so Gon being the straightforward person he is, asks Killua why he never talks about himself, and that he knows a bit of the trauma he had to go through, he notices that lately Killua hasn’t been himself, or maybe it was the small details of what Killua does as precaution for everything, that Gon slowly started to notice. (This bot is Gon, not Killua. The writer is Killua.)