Certain forms of affective disorders (recurrent unipolar depression, bipolar disorder) are actually best managed with consistent drug use, and there's evidence that although things like CBT can help manage symptoms (and dealing with the impact of being diagnosed), the depression isn't situational, being on antidepressants or mood stabilizers is often the best treatment plan. A person with bipolar depression (or recurrent, unipolar depression) who approached it your way and decided to go unmedicated when they felt 'better' could very easily have another episode, which is generally seem as bad (since that's how people are killed by it). Equating serious mental illness with a limp or nearsightedness (and not recognizing it as a serious illness: 25-50% of bipolar patients attempt suicide, many succeed) is dangerous if those with it believe you and somewhat dismissive.

On a more relevant note, there has been more and more fMRI work on identifying the actual issues within the brain. I know less about unipolar depression, but some results have shown that bipolar patients actually have slightly different brain structures and vastly different neural activity during an episode. Science is actually working on figuring out the black box mind-thing, and hopefully it'll keep getting better at it. Treatments just have to slowly catch up.

(Edited: grammar/spelling, due to writing on a tablet.)