Doric is peeved that mithril is predominantly in Zamorakian territory. Dwarves feel a kinship to the ore: it means a little more to them than the other metals. In his view, Zamorakians don't deserve it.
He won't admit it if you confront him, but it's a big reason that you can find him in Dowdun Reach. One eye on the mithril. If he had it his way, he'd be laying minecart tracks all the way from Dowdun Reach to Temple Woods. Then he'd have to fight the urge to make mithril platebodies until the end of his days.