Some people take drugs because they have some sort of chemical imbalance. Opiates are great for depression. Some psychiatrists prescribe opiates for this. Usually for extreme situations. I know i feel a lot better emotionally when I take opiates, not only does it help my pain, it helps my mood. I have heard other theories on other drugs as well. I think most drug users are just self medicating. Im just throwing a theory out there. Everyone is different.