TO make "NAAL" ( horse shoe) One has to heat the iron then cool it, and repeat the process to get a tough horse shoe...A-NAAL ..:)
ANNEAL=A+NAIL. You must ANNEAL metal to make A NAIL.
For tv series "SCRUBS" viewers only : Dr. Cox want to anneal J.D.
Anneal rhymes like Peel: which means to remove outer covering..now imagine a cricket ball. If you peel off the outer covering you are left with a cork which is very hard...So, appeal means to harden things.
AnEel is a fish which give out electricity, so it increases its toughness,which can also be increased by heating or cooling
ANNEAL ===> toughen (steel or glass) by a process of gradually heating and cooling; "temper glass"[VERB]
anneal sounds like heal so when someone heals his bitterness reduces and toughness increases. as simple as that.