Depression is treatable to those who want treatment.
Not everyone who seeks help gets what is right. Sometimes the wrong dose or wrong script is given, which happens because brain chemistries can differ between depressions and illnesses. Sometimes therapy can trigger. They say 95% of mental illnesses are treatable, many curable, but for that 5% where no God, drug or therapy can help them as their minds chemistry has twisted their thoughts. One can hope clarity is found that they are committed or heavily watched.
People who are severely depressed and/or are manic depressive/bi-polar are not able to think clearly.