Why do people suffer depression?

Some health conditions may cause depression-like symptoms. For example, hypothyroidism is known to cause a depressed mood in some people. Mono can drain a person's energy. When health conditions are diagnosed and treated by a doctor, the depression-like symptoms usually disappear.

Getting enough sleep and regular exercise often has a positive effect on neurotransmitter activity and mood.

It’s a mental illness. you can’t really control it.