Not sure why you think depression is not a severe mental illness. If your parents are depressed, this has very much the potential to affect you as a child. For one, since you got the same genes and the same risks involved and two, since you grow up in an environment where your two examples (mom and dad) are depressed with all the effects attached to that. Its difficult to give examples, but say you never see your parents smile or do anything with a smile. Don't you consider that actively malicious to their family members and broader society? Whereas a random heroin addict who sleeps under bridge and doesn't steal is just out of sight, out of mind. And what's to say the person isn't a heroin addict precisely because of mental illnesses?