What happens to a dwarf when his heart is weakened by gold and jewels in The Hobbit?
I think crucially the way to answer this question is to look at how Thorin changes when the company reach the mountain and when they gain the treasure. He becomes completely consumed by his greed of the treasure, not willing to part with any of it. He is willing to engage in a battle against impossible odds agains former allies (men and elves) to protect his treasure, even though he himself did not kill Smaug. He does not recognise that Bard might have some claim because it was he who killed Smaug. Also, he refuses to recognise that Smaug had added to the treasure of the dwarves from other sources, such as the men. Consider what the text says about this gold greed:
But also he did not reckon with the power that gold has upon which a dragon has long brooded, nor with dwarvish hearts. Long hours in the past days Thorin had spent in the treasury, and the lust of it was heavy on him. Though he had hunted chiefly for the Arkenstone, yet he had an eye for many another wonderful thing that was lying there, about which were wound old memories of the labours and sorrows of his race.
Thus, Thorin becomes blinded by greed which leads him to make irrational and illogical decisions, willing to court violence instead of peace as a means of keeping hold of treasure - some of which, at least, he does not actually have a claim on. Gold greed seems to weaken Thorin's heroic character, making him seem petty and selfish.