First off: I am not a doctor, merely a person who finds depression as a point of study, and had some first hand experience with the non-clinical kind myself.

Second: Please feel free to click the links as you read this. Depression is a very complex subject, and I've made a point to connect my terminology to relevant wiki links to try to avoid confusion and help educate. with that said:

The problem with the above post is that depression is an emotion. A completely normal emotion. It is a response to terrible life circumstances or situations. Even if someone is depressed for a extended period of time, they may not necessarily have anything clinically wrong with them if their life is continuing to go to hell. Often times psychotherapy may be used here to help people get back on their feet, and take control of their life again.

However "depression" can also be a more clinical problem, referencing Major Depressive Disorder, or any number of other mood disorders. It is important to note that these are all 'syndromes', not strictly 'diseases'. The key difference being a 'syndrome' is simply a collection of similar apparent problems, without a singularly defined source. This is where things get very messy.

Effectively, what Major Depressive Disorder is designed to catch and codify is a group of people who are more susceptible to the emotion of depression. Meaning, they clinically get depressed easier and for longer. Now, with an emotion as messy as depression, you can imagine how hard this is to measure and manage.

In context of above, there are multiple different possible causes for Major Depressive Disorder; unfortunately, due to the nature of emotions and depression, it's impossible to specifically say any one thing causes it. Once again, keep in mind this is about things which cause higher susceptibility to depression, not depression itself. Everyone can become depressed, this is normal, most people simply do not become depressed as much.

Now, as you can imagine, from how closely tied 'normal' depression and Major Depressive Disorder are, there's a lot of mis-diagnosis, and also a lot of pressure (monetarily, societally, etc.) for doctors to simply prescribe 'Anti-Depressants' to people to deal with it. These do solve the symptom: they make it harder to become depressed, but they do not solve the cause in the case of a normal depression; this can be a critical problem. Take the below example:

You are in a natural depression due to failing your classes, you are feeling cognitive dissonance about the subject: You feel all people who fail school are bad people; you are failing school; thus you are a bad person. You must change your feeling about failing school or you must stop failing school in order to relieve the feeling of you being a bad person. An anti-depressant will not fix this problem.

It is, however, worth noting, that since depression (of any variety) can commonly interfere with day-to-day activities, sometimes Anti-Depressants may help resolve problems like the above example due to letting you focus less on being depressed, and thereby get more done (in the example, do more studying). It is arguable both ways weather this is an acceptable use of Anti-Depressants, particularly in context of their many side-effects and addictive nature.

Anyways, I'll leave you with this short list-style take-away to try to help people on both side of the board understand each other.

For the non-clinically depressed:

• For you, getting out of a depression isn't too bad. Time heals all wounds, etc. For those with MDD, this isn't the case.
• Depression for most people is 'targeted' to a degree; you can at least often point to causes for why you feel like you're a terrible person or your life is shitty. For those with MDD, they often cannot point to why they feel depressed.
• For most, once they've resolved the problems causing their depression (failing classes, etc.), they are no longer depressed, or at least become significantly less so. People with MDD will often either not have a target for their depression, or feel the same/worse after resolving the subjects of depression.

For the clinically depressed:

• When other people tell you to 'snap out of it', or tell you why your life isn't terrible, they're not trying to make you feel like shit; for most people once they realize their life isn't so bad, and they have some control over it, they stop feeling depressed.
• When people say 'they were depressed once' they're not lying or bullshitting you, or were 'less depressed' than you necessarily; most people can get out of depression much easier, they may in fact just be trying to commiserate with you.