In The Hobbit: The Desolation of Smaug when the key hole is not found on the Lonely Mountain, why do Thorin and all the dwarfs get disappointed so quickly and leave Bilbo, while Bilbo remains positive?

This was the dwarfs' fight not Bilbo's, still he believed that there will be a way. This scene was really disappointing for me, because they worked with so much dedication and all the trouble they have gone through just to loose hope so quickly. Kindly help me understand this situation.