When it Comes to Depression - Does Anyone Really Know What They Are Talking About?

I started reading up on the condition that psychiatrists call depression and it was frankly, depressing.

I started my hunt by looking at my trusty Encarta dictionary, which said:
Depression - psychiatric disorder: a psychiatric disorder showing symptoms such as persistent feelings of hopelessness, dejection, poor concentration, lack of energy, inability to sleep, and, sometimes, suicidal tendencies.

Okay, that sounds pretty severe, right?

So, I started delving more into it and looked up depression on the Internet. It seems that some of the symptoms of depression are:

Sleeping too much, sleeping too little or a change in sleeping patterns; feeling irritable, sad or tense; loss of energy; decreased interest in things; restlessness or feeling slowed down; feeling worthless, hopeless or guilty; weight loss or weight gain...
Wow..it really sounds as if they are covering every possible ground here. "Hey, you lose some weight, you might be depressed! Oh, wait, you gained weight? Well, you could be depressed!"
I mean, half of the symptoms above could be explained by lack of sleep alone (haven't you ever felt irritable, exhausted, restless and uninterested when you don't have enough sleep?).

It seemed that trying to find symptoms that made depression easy to figure out was impossible, so I searched more into depression and conversely, into happiness, and this is what I found:

a) There is no consistent definition of depression out there. It's more like, if you're not happy, your depressed, and only your doctor can delineate when that happens.

b) Even a few weeks of sadness can be considered depression! What's with that craziness? If your mom died, are you supposed to be "okay" with it in a couple of weeks? I don't know how long it would take me to bounce back - but it certainly wouldn't be weeks.

c) Reading up on depression and how to "get happy" is actually depressing. If a person is just moderately happy and wants to attain higher states of happiness, and they read up on it, it seems as if everyone on planet earth falls short of the standards of happiness now required.

d) For some reason or another you are supposed to "live with depression". That's more depressing than being diagnosed with depression. No one should have to "live with" anything in their life.

I think I can officially say that there seems to be no finite information on depression available. This makes me wonder, if doctors and psychiatrists don’t understand this condition, how could they possible come up with a plausible cure for it?