Why do so many people believe that jealousy is the root cause of all evil, and what are some real-life examples of it causing problems?

Is it weird to date my ex’s friend? I’m 16, and our relationship only lasted a couple of months. After liking said guy for a while, I recently found out he and my ex knew each other. Would it be wrong of me to still pursue him?
Why do people feel jealous even when they’re aware it’s irrational? What can be done in those moments of awareness?