“A merry heart works like a medicine” (Proverbs 17:22)
Unforgiveness is now a recorded medical condition, and generally causes chronic anxiety. When we do not forgive, we carry around a toxic burden of anger, hatred, revenge and dislike. These negative emotions create a toxic cocktail of adrenalin and cortisol, which impact our immune system.
Recent studies indicate that 61% of cancer patients have unforgiveness issues.
On a metaphysical level this fits in with the concept that we are all one. The anger/hatred/revenge/dislike is being heaped on ourselves, not just those whom it is aimed at, with negative impact.
(The Holistic Guide to Hypnotherapy by Steve Webster CHt and others. Page 127)