Dude, depression has very real clinical basis.
http://en.wikipedia.org/wiki/Major_depressive_disorder#Biological
Serotonin is hypothesized to regulate other neurotransmitter systems; decreased serotonin activity may allow these systems to act in unusual and erratic ways.[31] According to this "permissive hypothesis", depression arises when low serotonin levels promote low levels of norepinephrine, another monoamine neurotransmitter.[32] Some antidepressants enhance the levels of norepinephrine directly, whereas others raise the levels of dopamine, a third monoamine neurotransmitter.
You don't think those drugs just work magic, do you?
Serotonin levels can be measured. Unfortunately, this usually involves an autopsy of the brain. This was how they started developing drugs for depression in the first place -- they looked at slices of the brain under a microcope and chucked some of it a test tube and whoops, it looks all fucked up. People who are severely depressed and it seems no amount of happy thoughts help have really, really low levels of serotonin. What can we do to make it less fucked up?
http://www.webmd.com/depression/features/serotonin
Although it is widely believed that a serotonin deficiency plays a role in depression, there is no way to measure its levels in the living brain
So, rather than wait for the patient to die and cutting open their brain, doctors have to use other ways of diagnosing it -- symptoms. It isn't perfect, though.
Is it something that one can overcome with self realization,spirituality or just will power
Can you add serotonin in your brain with willpower?
In a roundabout way, there _are_ methods of increasing serotonin. Pscyhotherapy and exercise werks, but not always.
For people with genuine imbalances of brain chemistry, no amount of positive thinking will help.
While depression is over-diagnosed, it EXISTS in the same way diabetes exists.