Sadness/depression. Just as with adults, the cause of depression is not fully understood in adolescents. There are biochemical theories hypothesizing about seretonin, norepinephrine and Dopamine and psychosocial theories that environmental facotors cause depression. Some suggest that diet and sedentary lifestyle are causes of adolescent depression. Hereditaey or genetic causes also continue to examined.
Same as older. Young people experience the same stresses and struggles with their lives that we older folks do. They struggle with school and social demands, family issues and other dimensions. They may have predispositions in their family history too.