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Right from the very beginning of An Unexpected Journey, Thorin is a fairly abrasive character. As The Hobbit trilogy develops and expands, this is shown to be in part because he is a fierce and proud leader, and in part because he has lived through some truly harrowing experiences. His life has made him very guarded, and very particular about the company that he keeps. Thorin makes several rash, bold and unwise decisions throughout the trilogy, including mocking the king of Mirkwood and rejecting the deal that Thranduil offers to let the dwarves out of the elven prisons, as well as leading Fili and Kili to Ravenhill, where a trap lies in wait and they all die.
But one of the worst decisions that Thorin makes, one that really brings out some ugly and unfavorable sides of his personality, is his obsession with the Arkenstone. He is willing to forsake alliances, lose friends, and let an entire race of people suffer for it. When Thorin does succumb to the dragon-sickness, after forcing Smaug out of the mountain, he becomes all-consumed by the mighty wealth of gold, and by finding the precious gem of his people. He is essentially blind to the suffering it is causing the people all around him, including his most loyal companions, especially Bilbo. But interestingly, it may be partially Bilbo’s fault that Thorin falls and sinks so deeply into dragon-sickness so quickly.
It is well known that Thorin’s family has a pre-disposition to the sickness. Thorin’s grandfather Thror essentially amassed a pile of gold so vast and greedy that it drew Smaug out of the mountains and into the halls of Erebor, where he slaughtered all the dwarves who dwelled there, as well as most of the nearby city of Dale. Thror’s lust for wealth, combined with his hubris at thinking that the Arkenstone made him untouchable and beyond reproach, lead to the downfall of his kingdom, and all the sorrows that befell the line of Durin afterwards.
Knowing this, it's clear that Thorin clearly has this already within him, as is confirmed by Balin when he tells Bilbo:
“Dragon-sickness. I’ve seen it before, that look. It is a fierce and jealous love. It sent his grandfather mad.”
There are many parallels made between Thorin and his grandfather. The fear of the same weakness running in his blood, just as Aragorn has with his fear of Isildur’s weakness if he takes the throne of Gondor. And in the end, everything that Thorin and his companions fear comes true. Thorin does indeed fall to dragon-sickness, and loses all reason and shred of compassion in the process.
But even for Thorin’s bloodline, who clearly have an Achilles heel when it comes to dragon-sickness, Thorin falls remarkably fast. It takes a mere matter of days for him to go from to brave, noble and stirring leader of the group to a selfish, guarded, paranoid, isolated version of his former glory. And the speed for which he succumbs to the sickness can only be explained by two things. The first is that Smaug is a particularly large and malignant dragon, and has brooded over the gold for several decades, which would make the curse particularly potent. The second is that there is another, even darker force that is guiding him towards ill: the One Ring.
Earlier in their journey, Bilbo picked up the evil object in the goblin tunnels underneath the adjacent mountain, and used it to escape his terrifying game of riddles in the dark with Gollum. The ring remains quite successfully hidden in Bilbo’s possession, and the others are barely even aware of its existence, especially in the films. However, the ring does have a way of seeping darkness and despair into everyone who comes into contact with it. It has a special ability to poison people’s minds, to manipulate and twist the mood just as it does at the council of Rivendell, where everyone begins arguing over its mere presence.
This fact, paired with Thorin’s already pre-disposed nature towards dragon-sickness, would explain why Thorin falls quite so quickly. It would provide an explanation as to why he is incapable of seeing the awful cruelty he is showing to those who have been most loyal to him. He is not only back in the mountain full of the trauma of his murdered people, sifting through a pile of cursed gold to find a stone that could have been swallowed by the dragon years ago. He is also in the presence of the One Ring, whose entire purpose is to turn people towards the most dark and hopeless path in their future. In this way, Bilbo may have tragically and unwittingly brought about the awful fate of some of his dearest friends.