Adolescence is famously described as a period of "Storm and Stress" (by G. Stanley Hall). During this time:
Hormonal Changes: Fluctuating levels of hormones lead to rapid mood swings.
Brain Development: The limbic system (the emotional center) matures faster than the prefrontal cortex (the logic/control center), meaning teens feel emotions intensely but have not yet developed the "brakes" to stabilize them.
Identity Crisis: The struggle to find oneself leads to frequent feelings of anxiety, irritability, and uncertainty.