Because he doesn't believe that someone has defeated Smaug & that he is dead.
Gandalf is worried that Smaug may become an ally to the newly returned Sauron.
Thorin vows to kill anyone who withholds the Arkenstone .
Thorin Oakenshield, son of Thrain, son of Thror, King Under the Mountain.
Thorin Oakenshield is a dwarf. He normally wears a sky-blue hood and has a beard. He might have a resemblance to Gimli (Gimli being Gloin's son, and Gloin is the third cousin of Thorin)
Thorin Oakenshield