Most people take time to come to terms with stressful events, such as bereavement or a relationship breakdown. When these stressful events occur, your risk of becoming depressed is increased if you stop seeing your friends and family and try to deal with your problems on your own.
You may be more vulnerable to depression if you have certain personality traits, such as low self-esteem or being overly self-critical. This may be because of the genes you've inherited from your parents, your early life experiences, or both.
If someone in your family has had depression in the past, such as a parent or sister or brother, it's more likely that you'll also develop it.
Some women are particularly vulnerable to depression after pregnancy. The hormonal and physical changes, as well as the added responsibility of a new life, can lead to postnatal depression.
Becoming cut off from your family and friends can increase your risk of depression.
Alcohol and drugs
Cannabis can help you relax, but there's evidence that it can also bring on depression, particularly in teenagers.
"Drowning your sorrows" with a drink is also not recommended. Alcohol is categorised as a "strong depressant", which actually makes depression worse.
Head injuries are also an often under-recognised cause of depression. A severe head injury can trigger mood swings and emotional problems.
Some people may have an underactive thyroid (hypothyroidism) resulting from problems with their immune system. In rarer cases, a minor head injury can damage the pituitary gland, which is a pea-sized gland at the base of your brain that produces thyroid-stimulating hormones.
This can cause a number of symptoms, such as extreme tiredness and a lack of interest in sex (loss of libido), which can in turn lead to depression.