If we ignore the literal reference to the heart and focus on the metaphor, the emotive based felt experience is biologically predisposed to parse the bad and remember the good. Do you see what I am saying?
The light is a heavy burden to bear when your character of virtue is surrounded by darkness. Sustainable for sure, but the economies of ideal ideas need no idols. Have you heard the analogy of the cave? Better yet, have you seen the light?