The problem is that the decision-making vessel is the thing experiencing the illness, so it's not always so simple as "Oh wow I'm ill, I should see a psych!" In addition, treatment for mental health is significantly less reliably effective than treatment for something like diabetes. If you find yourself questioning whether you should talk to someone, you absolutely should; my quarrel is with the way your statement seems to imply (to me) that diagnosis and treatment of mental illness is simple.