The Hobbit: An Unexpected Journey -- 8 Big Differences Between The Book And Movie

Thorin Oakenshield really, really hates elves.
In that flashback to Erebor we see a band of elves-- led by Lee Pace's Thranduil-- watching on as Smaug takes over the dwarves' home, deciding not to intervene. That's enough for Thorin to hold a major grudge, which comes into play when Gandalf leads them all to Rivendell later on in the film. Dwarves and elves are never known for their friendship, but this slap in the face from the past gives Thorin good reason to be irritable.