They grew up together. They've been together long enough to have played with the gardener when they were young. Thee still thinks of a packed lunch as something he will never be able to repay him. Thee has obviously watched Tanthai be pushed to the place he is now, watched him go from someone who changed a fellow child's life with a packed lunch and played with the gardener to an abused, damaged and lashing out young man who is simply so hurt he feels empty.
Thee cannot ever pay back a packed lunch but he also can't run with him. Because running with him risks losing him forever. If they run together and get caught, what happens? They lose. They lose everything, even this painful time together and even Tanthai's anger and drinking and pain. Thee would be gone and he'd be unable to be any kind of presence, to check on him, to look at him and be there to try to defend him.
I think the most painful part is that Tanthai desperately seeks out comfort from Thee that he can only ever half get, that Thee cannot give him the comfort he wants both because of the chains on them from his father and because Tanthai has been too hurt to accept what Thee feels he can offer. And the half-measures of comfort only hurt more in the end and so Tanthai hurts Thee to try to stop the pain, hurts everyone around him to try to stop his own pain.
(Tanthai would fall over if Thee just pulled him into a hug and didn't get go when he started to protest but Thee can't do that because he is the servant, he is the one below him, he has no power and Tanthai tries to wear his golden chains as power the best he can.)
Tanthai is as absolute asshole and constantly getting himself in trouble to prove the point his father is constantly making. If what he does doesn't matter than he might as well just do the worst things possible because at least it gets him something. And Thee is always there. Thee never leaves.
That's why the preview for next week hurt so much. Because Tanthai believing that Thee would leave him if he wasn't being paid is the ultimate proof of how much he's been hurt. He's screaming out for any kind of reassurance and this time Thee has to give him something.
Thee has never repaid that packed lunch. Thee has never forgotten them playing together. Thee has never left Tanthai's side. Thee is the last and frayed link Tanthai has to his humanity and Thee cannot let that link break. Or he will lose the person he loves the most, the person he owes the most.
The history between them, that we know so little about, is the key to what really happened that night and Thee is the only one who can truly tell us what happened... and he will have to do it to protect Tanthai.