In The Hobbit, whose fault is the ancient feud between the dwarves and elves?
As in most ancient feuds, both sides believe that they are in the right and that the opposing side is the instigator of the feud. The dwarves and elves are no exception to the rule. Their feud dates back to ancient times in which the elves "had some wars with some of the dwarves, whom they accused of stealing their treasure" (152). Of course, the dwarves gave a completely different version of the story, saying that "they had only took what was their due, for the elf-king had bargained with them to shape his raw gold and silver, and had afterwards refused to give them their pay" (152). Both sides viewed the other as being completely at fault and refused to accept the smallest modicrum of guilt in the entire affair.
When the elven-king captures the dwarves snooping through his realm, it matters little that "Thorin's family had nothing to do with the old quarrel" (152). The old prejudices still remain, and the elven-king's bias does not make him sympathetic to the dwarves. Both sides' actions reinforce the old stereotypes; the elven-king acts haughty and unforgiving, and Thorin and crew are tight-lipped about their quest and seem suspicious. Hence, the feud continues.
Interestingly enough, the elf-dwarf feud will continue until the War of the Ring in The Lord of the Rings when Legolas and Gimli's friendship at last lays the old feud to rest.