What's the natural cure to an old man's inability to forgive people?
Alzheimer's.
Old Man's Cure
Explanation
Explain Bear
Listen up, smarty pants! The joke is about an old man who can't forgive people. The "cure" is Alzheimer's, which makes him forget what he was mad about in the first place. It's funny because it's dark and it's basically saying old people lose their memory!