Cheap meaningless sex works for me. Drugs where required might only be needed temporarily, not unlike how a muscle relaxer can stop the spasm to break the pain/contraction/pain cycle and give the back some time to heal. Good point on differentiating sadness from depression.
Causes could be chemical (such as bipolar), environmental (poverty and hunger), situational (mourning, break ups), etc., or mix and match. And then depending on what you are dealing with, along with your resources (intellectual, financial, access), depth of affliction and given whatever effective methods available, etc., is how you deal with it.
If a severe chemical imbalance then psychopharmacology might be required. But even if it is something like manic-depression aka bipolar, if not severe, then maybe psychotherapy can help a person learn how to control and ride out the swings.
I'm not sure how accurate it is to say depression is so common in society because more likely it's probably simply pretty common to humanity and I would think it occurs with regular frequency throughout much of the animal kingdom, particularly of the higher functioning species. Though of course stresses of society can exacerbate while tools of society can alleviate.
Now, when you say your parents say chemical but you say emotional, realize that the brain is neurochemical. The brain is made up of neurons. So emotions in that sense can be described as chemical.
Depression can be so severe that it does drain energy or prevents a person from accessing their energy (however you care to describe it) possibly because it seems to use a lot of energy, exhaustingly so, leaving no energy for other tasks. And if that interferes with the tasks of daily living, then you've a serious problem, seek professional help.
As to your comment that this so-called energy "is no longer able to support the amount of stress they receive", realize that even while stress can bring on depression, the depression itself is stressful, it generates stress, so much so, that a person can die of a broken heart. Of being so sad, so stressed out, that the heart gives out. And that's this: http://en.wikipedia.org/wiki/Takotsubo_cardiomyopathy
Ego issues from either direction can lead to depression, so, as you say, when lacking confidence in yourself, worried about your importance, sure, that would be depressing. But also someone's overly inflated ego probably leads to some not very pretty mood swings too. Though while lacking self importance probably more likely leads to some depression or at least melancholy, too much self importance surely leads to delusion.
And looking at it from that aspect, then meditation might be helpful. Learning to understand yourself and what makes you tick. Because the more you know yourself and the more practiced you are at centering yourself then the better your chances to smooth out some of the mood swings of life.
And there is research now in that direction. And here's that...http://www.tibet.emory.edu/
During the 2003-2004 academic year at Emory University, increasing signs of mental distress among undergraduates, including several suicides, prompted Emory student Molly Harrington to ask whether there were resources available to help young people deal with stress and depression. Based on her appeal, Geshe Lobsang Negi developed CBCT, a model for a secular compassion meditation practice drawn from the vast, varied Tibetan Buddhist tradition. In 2005 the Emory-Tibet Partnership and the Emory Mind-Body Program forged an innovative study to examine the impact of compassion meditation in treating depression among undergraduates.