Jealousy is the negative emotions ideas or acts triggered when someone believes a rival or third party is threatening an important relationship. When you become a teen it's as if someone rewrote all the rules overnight and there's a pop quiz on them every day. Suddenly fitting in seems to matter more so you check out what your friends and people at school are wearing and what's on their MP3 players. You also face a bigger and more diverse mix of students so you encounter more competitive and cliquish groups than before.