Depression/ mental illness is often hereditary.
Most of the people you see on the news doing crazy things are likely to have schizophrenia, or other issues. If you happen to be pre-disposed, taking drugs (usually marijuana), can trigger life-long schizophrenia. People with schizpophrenia start taking drugs , like ice, to stop hearing voices and it makes them worse. At that point, their life - and their family's life is pretty much f^%cked unless they can make huge changes.