Why is Cassius jealous?

Cassius is jealous primarily because he feels overshadowed by Julius Caesar's rising power and influence in Rome. He believes that Caesar's leadership diminishes his own status and capabilities, leading him to resent Caesar's perceived superiority. This jealousy fuels Cassius's desire to conspire against Caesar, as he fears being eclipsed and seeks to restore his own sense of agency and importance. Ultimately, his envy drives the plot against Caesar, highlighting themes of ambition and insecurity in the play.