Of all of the many disliked characters in Peter Jackson’s movie adaptations of The Hobbit, Thorin is probably the one who gets the most unjust hatred. It is true that Thorin can be too proud and stubborn, and often appears to be putting his lust for gold above the people he has promised to help, including the company of loyal companions who have stayed by his side. However, despite his flaws, the dwarf is often saddled with an unfair perception.
Many members of the audience, when watching The Hobbit movies, believe that Thorin is selfish in leading the other dwarves back to Erebor. It's easy to believe, upon watching the films, that that the fight to reclaim their homeland is only a guise or a compelling excuse to give Thorin access to the hoards of gold. But actually, despite his ignorant and arrogant claim to the throne as his birth right, it isn’t he who started the quest in the first place.
This is only touched upon briefly in the extended edition scenes, and expanded upon much more effectively in the book, but it was really Gandalf the Grey who set the wheels in motion for the quest for Erebor. The movie depicts the scene of Gandalf approaching Thorin in the tavern at Bree, and convincing him that now is the time to go back to the Lonely Mountain:
“I urged your father to march upon Erebor, to rally the seven armies of the dwarves. To destroy the dragon and take back the Lonely Mountain. And I would say the same to you. Take back your homeland.”
Thorin at first thinks this is folly and resists the wizard's urging, but is eventually swayed by pride and by desire for revenge of his fallen kin, who lost their lives when Smaug the dragon first invaded their kingdom.
The question remains, why would Gandalf set the dwarves on such a dangerous quest, knowing that there was a very high risk they would all die along the way? The answer has to do with the strategic and political position of the Lonely Mountain itself. When looked at on a map, the Lonely Mountain is a gateway to several of the surrounding lands, which makes it a very important place for the future fate of Middle Earth. Just below the mountain is, of course, Mirkwood, a vast expanse of forestry in which Thranduil’s elven kingdom lies. At the base of Mirkwood is Dol-Guldur, the stronghold where the white council stand off against Sauron’s shapeless necromancer form and his ring wraiths. And above the Lonely Mountain, to the North Westm is the stronghold of Gundebad (where Tauriel goes with Legolas after healing Kili in Lake Town) and the lost kingdom of Angmar. Both of these locations held significant dark powers in the Second Age of the world, when Sauron was first rising to power in the footsteps of Morgoth.
In essence, the Lonely Mountain is the heart between the ancient powers of the north, and the growing darkness in the south. If the Lonely Mountain were to fall into the wrong hands, then the evil would pour forth out of Angmar and Gundebad, seep down through Mirkwood and spread all the way to Dol- Guldur. In the end, this would essentially give Sauron dominion and control over the entire eastern hemisphere of their world. Gandalf knows this, and he knows that the only way of stopping the dark lord's power from growing is to take Erebor before he can. The best way he can do that is to lead the dwarves in a quest to take it back. In a continuation of their conversation at the tavern, Gandalf explains:
“The Lonely Mountain troubles me, Thorin. That dragon has sat there long enough. Sooner or later, darker minds will turn towards Erebor.”
This leads Thorin to the realization that there is one who lies in Erebor who would give Sauron unbeatable strength and make him near impossible to contain: Smaug. The dragon has lain dormant and undisturbed within the walls of Erebor for decades, but it is only a matter of time before he wakes, and becomes aware of the bid for power that is collapsing alliances all around him. And if Sauron were to strike up a deal with a fire-breathing dragon, he could obliterate the face of the earth in no time at all, by burning those who choose to resist him into submission.
So when Gandalf sets up the quest for Erebor, he understands that some of the dwarves' lives may be lost along the way. He also understands that there are far more drastic things at stake, and that if Erebor falls into Sauron’s hands, the world is doomed. Thus, he is willing to endanger the lives of a few to save the lives of many. And so is Thorin, once he realizes the danger that Middle Earth is in, and that he, Fili and Kili (who are of the bloodline of Durin) are the only ones who can reclaim the mountain and stop the evil from spreading.