[LOTR] Why was mining so deep in Moria considered greedy?

Wasn't mining further down just being productive use of their mine and resources and encountering the Balrog just bad luck that they couldn't have predicted? Were there other indications that going that deep was a bad idea which the dwarves intentionally disregarded to get more wealth due to greed? If not. how does one define the limit of reasonable mining and when it turns greedy?

"they delved too greedily and too deep, and disturbed that from which they fled, Durin’s Bane"