I am depressed and i dont why?

Go get a checkup with your doc. Once you are found clear of a health issue that might cause such feelings they can refer to a mental health professional for assessment.

Medications, endocrine imbalance (thyroid issues), sleep disorders, prior use of recreational drugs or alcohol, should be considered, initially. If the evaluation is negative, a psychiatric or psychological assessment is appropriate. Medications and/or therapy should reverse most or all of the symptoms.