 Gandalf persuaded him that stealth, rather than force, was needed to infiltrate Erebor; they would therefore need a burglar.Gandalf feared that Sauron could use Smaug as a weapon, and was concerned that Thorin's pride and quick temper would ruin the mission to destroy the dragon.He thought that Bilbo would be a calming influence on Thorin, as well as a genuinely valuable addition to the company.