Why was Ganelon very angry at Roland?

Ganelon was very angry at Roland primarily due to feelings of betrayal and jealousy. He resented Roland for being chosen as the commander of the rear guard during the battle against the Saracens, believing that it undermined his own status and authority. Ganelon's anger was further fueled by his desire for revenge, leading him to conspire against Roland and the Frankish forces. This animosity ultimately set the stage for the tragic events of the "Song of Roland."