The GP is well placed to check these things out. Abuse People with learning disability are at risk of being neglected and physically or sexually abused, because they cannot easily protect themselves, or may not be able to tell other people what has happened. Abuse may lead to depression. Personality may play a part in depression with some people seeming to be more vulnerable than others, perhaps because of the way they were raised or because they were born with a depressive tendency. Generally speaking, women get depressed more often than men.
Hormones probably play a part, as with monthly mood changes before each period.
Some people who get depressed will also have times when they are elated and overactive. This form of depression, known as manic depression, tends to run in families. First you have to be aware that your friend or relative may be depressed. The time to get help is when any changes in behaviour, withdrawal or gloom have gone on for some weeks. Then you should seek professional help.