He was interested in the idea of ISIS and wanted to meet his friends who were fighting for Islam there
He felt bored and empty, and wanted a challenge that would make him seem cooler and feel good about himself
He learned that men must do jihad, and believed ISIS was the land of men. Thus going to ISIS would help him achieve his goal of doing jihad
As a Muslim, he was obliged to go, because ISIS provided a life with real Islamic rules as a caliphate in the past