Where does Leontes' jealousy spring from? Is it spontaneous or has it been built up for a long time?
In William Shakespeare's The Winter's Tale we find Leontes suddenly breaking out in a sudden, and what seems to be uncommon and rather unnecessary, jealousy fit. This occurs after his wife, Hermione, suggests to Leontes's childhood friend, the king Polixenes, to extend his visit for a longer period of time.
What is usually assumed from his reaction, is that Shakespeare purposely added it to denote from the very beginning the natural weakness of Leontes's nature. This will serve as foreshadowing for the number of actions through which Leonte will show a very suspicious, and uneven, psychological state.
It is also a good technique to expose the deep contrast of Leontes's personality compared to everybody else's. Hence, yes he is jealous when he reacts this way, but he is his inherent weakness.