There is no one thing that causes depression. It’s a mixture of genetics and environmental influences. The outdated view from some people that you just need “get over it” or “sort yourself out” should just be disregarded.
Environmental
Life events that typically cause depression symptoms include:
- bereavement
- sickness
- relationship issues
- money worries
- job worries or redundancy
- substance abuse
It may well be that one of these events start changing your behaviour which causes other to become more likely to occur and cause a chain reaction. The same goes for turning to substance abuse to get a momentary relief only for the long term effect being to make the depressive symptoms worse.
Biological
There is familial link to depression, as someone with a family history of depression is more likely to experience depression themselves.
Other biological connections include a enhanced vulnerability to depression following giving birth, in the form of postnatal depression.
Diseases are also known to increase risk of depression. This includes hypothyroidism, cancers, heart disease, as well as head trauma.
