How do you explain Leontes' insane jealousy in The Winter's Tale?
This is an excellent question. Since Shakespeare explored jealously in Othello, it appears that jealousy is not what is important but the results of that jealousy is what he wanted to explore.
Several years ago, I was fortunate to be in a class where the actor Patrick Stewart, who played Leontes in a Royal Shakespeare Company production was a class guest. He told us that he was having trouble understanding Leontes and as an actor, needed to understand why Leontes "out of the blue" so to speak becomes insanely jealous. He consulted a psychiatrist friend and asked him about it.
The result was a discovery that Shakespeare has delineated a real mental illness. Each step of the illness, which can be confused with jealousy, happens to Leontes just as the real disease progresses.
Shakespeare was a great observer of his fellow man and must have observed someone behave in this manner and filed away this knowledge for future use.
Hope this helps.