"Now, if you kill an enemy, be sure to eat their heart - to gain their courage. Their rich, tasty courage." __ Professor Farnswroth, Futurama, sending Fry off to war.
I always considered the heart containers and heart pieces to be more of a "gameplay device" than anything else. Sure NPCs will say interesting things sometimes when you get a heart piece (Hena at the fishing hole in TP for instance if you fish the heart piece out of the rock. "Whose heart did you steal? You sweetheart, you!" ), but I mostly consider the heart pieces popping out of bosses and stuff to be mainly a visual representation (for us players) of Link "gaining strength." Maybe, though his eyes, he's not grabbing hearts, he just feels victiorous and stronger than he did before, for having defeated a big evil creature.