Thorin becoming explicitly possessive over Bilbo during his time goldsick makes sense.
Thorin is a dwarf carved by his denial of self in his pursuit for reclamation of the mountain. He is reserved and forces himself into a single mindedness that would have driven him mad eventually with or without the gold.
The ire and bitterness he originally directs towards Bilbo is not, of course, actually meant for the hobbit. It stems from Thorin's desire for what he thinks he cannot have, Thorin having amputated the part of him that wants. The part that wants with an ache that Thorin cannot feed, cannot entertain until the mountain is his.
When the goldsickness takes Thorin, it is an undoing that Thorin would have been horrified to witness in himself were he not so swept away by this taste of paradise. To wallow in his want and to take what he has felt he always deserved. To put himself and his desires first. The rush of comfort, then pleasure, then angry hunger and paranoia of loss. It is more instantly addictive than any drug known in Arda.
Of course Thorin always deserved what he fought for. Even a traitorous snake like Thranduil would be hard pressed to deny such a fact. But now, goldsick and sweating and ravenous in his barking orders, he goes too far. Allowance becomes indulgence and Thorin finds that his desires are in fact, bottomless. He takes without reservation, no longer able to tell himself no.
What did this company of thieves do anyways, to deserve a handful- let alone a fourteenth!- of his wealth?
What had they done, when Thorin had still felt so alone all these years, the burden of their people and their home and his grandfather's treasure weighing on his shoulders?
He always was, wasn't he? The only one Thorin carefully avoided to touch, even a brush of their hands while walking would have been too much of a distraction. The one he didn't often speak directly to, but always keenly leant an ear across the campsite. Ever the voyeur, because if he took directly he would not be able to let go. Because the hobbit would only drive him to distraction.
He no longer needed to repel himself from the hobbit. Not here, not in his mountain. With or without the Arkenstone, Thorin felt it in his blood that the mountain belonged to him. Felt it welcome him home after regurgitating the filth that took dragon shape.
Thorin felt delirious, high with greed. From Bilbo's bronze curls that have since grown shaggy, from his legs thickened from journeying, from his soft speckled cheeks which demanded but a brush of Thorin's lips-
Thorin would dress him in golden and silver sapphire studded chains around his pretty neck and down to his ankles. Precious jewels and beads and bangles until the hobbit was heavy with them. So heavy- so beautiful- that the hobbit would not think to leave. He would give Bilbo everything.
He was acting like the dragon that had just fell. The dragon, that was currently rotting in the lake of Laketown. Thorin too, was rotting. Thorin too, felt like a dragon. Were he a true dragon, he would fly back to Bag End to burn it down. No more would there be this flippant debate about what Bilbo would do, or where he would go after Erebor was restored.
No longer would Thorin allow room for the possibility of loss. Never again would he face the pain of abandonment or the agony of failure.
The ultimate culmination of the goldsickness was the loss of Thorin's true goodness. The true heart that was lost in that mountain, that which the company still desperately searched for.
