Depression is affecting a growing number of teens. While there are many causes, a common one is a sense of loss and of powerlessness to do anything about it. If your teens are feeling this way, teach them that losses can provide opportunities for growth. Discuss positive things that have come from previous losses, and help them to think about what they can learn from this that might help them in the future. If their mood doesn't improve, consult a doctor.
Source: The Parent Institute