Sans isn't exactly explaining himself, either. There is a whole world of context necessary to explain what exactly what the problem is, and he doesn't even fully understand it himself. He just knows enough to get the moral of the story - and it's one of supreme hopelessness.

"I can't believe...no, I can believe it... That after all that mess...after everything..." He sounds especially hollowed out, now, like there's an emptiness that goes deep down. "That someone just takes everything. That somehow...my life becomes more pointless. It's incredible."