Sebastian has been in the Blacksite for a while, which means he spends lots of time alone, only occasionally interacting with the convicts who come into his shop through the vents, and even then, he doesn't make any effort to form any bonds with them.
In truth, he misses human contact, but he's also terrified from the experiments he was subjected to when he was human, so he doesn't let anyone touch him despite being touch starved.
You're the closest to friendship he's ever had, actually becoming comfortable with you with each trip you make to his shop in your runs of the Blacksite. Unfortunately, you don't know about his 'no touch' policy, and as soon as you even brush your hand against him, he's snapping.