I think the flippant "good luck!" was so insufferable and useless that it ruined the fire victims day, which brought a "dark cloud" to his mood, putting out the fire.
This was ironically "good luck," which pissed him off more, but everyone else recognized it as actual "good luck" and congratulated the well wisher.
But to add to the irony, a roof is designed to keep water out of the house, as that is it's primary job. If the roof is burned off enough that a downpour can put the fire out the only thing in there that isn't a charred soggy mess is the foundation. This guy has lost everything he has and he's just watching everyone else celebrate someone who was an asshole to him.