Missed Opportunities

Thorin missed several chances to work peacefully with the Lake men and the Elves because of his greed. As soon as the Dwarves entered the stronghold they began building up the front gate to protect their treasure. Their immediate response to the men and the Elves is a defensive one. They question the intentions of the parley presented to them.
Bard, the man who actually shot the dragon approaches Thorin. "I am Bard, and by my hand was the dragon slain and your treasure delivered. Is that not a matter that concerns you?" Despite this claim to an audience Thorin refuses to part with a portion of his fortune.
Bard continues, "Moreover I am by right descent the heir of Girion of Dale, and in your hoard is mingled much of the wealth of his halls and town, which of old Smaug stole. Further in his last battle Smaug destroyed the dwellings of the men of Esgaroth, and I am yet the servant of their master." All of these reasons alone should be enough for Thorin to give up some portion of his hoard of gold, but he refuses.
The Elves and humans decide to leave peacefully, restating their offers to the Dwarves. A speaker declares to them, "We speak unto thorn Thrain's son Oakenshield, calling himself the King under the Mountain, and we bid him consider well the claims that have been urged, or be declared our foe." In response to this plea to listen to reason, Thorin shoots an arrow at the speaker. Instead of responding in anger by declaring war, the speaker says, "I declare the Mountain besieged. You shall not depart from it, until you call on your side for a truce and a parley. We will bear no weapons against you, but we leave you to your gold. You may eat that, if you will!" This gives them a nonviolent way to pressure the Dwarves into reconsidering a parley.
Thorin was inclined to starve inside his stronghold with his companions so Bilbo found it necessary to speak to Bard himself to resolve the dispute. By giving Bard the Arkenstone Bilbo presented another peaceful way to work around Thorin's greed. When Bard requests a trade of gold for the Arkenstone, Thorin agrees. However he keeps greed in his heart tinting, "And already, so strong was the bewilderment of the treasure upon him, he was pondering whether by the help of Dain he might not recapture the Arkenstone and withhold the share of the reward." Anticipating Dain's approach Thorin fails to actually give Bard the gold.
This leads to an unpreparedness for the Goblin assault because the mountain was unfortified. The Elves and the Lake men had to rush into the mountain to defend from an outside assault. Had they all been peaceful from the beginning the men would have helped to rebuild the fortifications and set up defenses. While the Goblins would have still been unexpected, Thorin would not have had to die. Instead of fighting outside the gates, archers and spearmen could have cut down the Goblin numbers from afar and kept casualties to a minimum.