Question

what is the basic cause of depression

Answer 1

Problems such as social isolation due to other mental illnesses or being cast out of a family or social group can contribute to the risk of developing clinical depression. Serious illnesses. Sometimes depression co-exists with a major illness or may be triggered by another medical condition. Substance abuse.

Answer 2

Social isolation/ exclusion, family problems, peer pressure, abuse, bad past, insecurities are a few causes.
For me, it's anorexia and suicidal thoughts and tendencies.