1. Romeo's Jealousy of Paris: Romeo becomes jealous of Paris when he learns that Juliet is to be married to him. This jealousy leads Romeo to crash the Capulet ball, where he meets and falls in love with Juliet.
2. Juliet's Jealousy of Rosaline: Before Romeo meets Juliet, he is infatuated with Rosaline. Juliet becomes jealous of Rosaline and wishes that she were as beautiful as her.
3. Tybalt's Jealousy of Romeo: Tybalt, a hot-headed Capulet, is jealous of Romeo because he is a Montague. This jealousy leads Tybalt to challenge Romeo to a fight, which results in Tybalt's death.
4. The Montagues' Jealousy of the Capulets: The Montagues and the Capulets are two feuding families. The Montagues are jealous of the Capulets' wealth and power, while the Capulets are jealous of the Montagues' influence and political connections. This jealousy fuels the ongoing conflict between the two families.