Your friend obviously has an emotional interest in seeing depression as somehow not real. These issues tend to become matters of 'belief' rather than a logically based opinion, leave him to it and if he tries to argue with you point him towards any respected psychiatric journal and continue your day.
The current theory on most clinical disorders in psychiatry is the 'stress-diathesis' approach, which is that a biological tendency towards a condition is set off by 'stress' (something bad happening) in the environment. Chemical inbalances are certainly part of the picture, but not necessary considered the main issue any more.
The DSM IV-TR gives a wide range of symptoms which taken together could constitute a diagnosis of depression. If you wanted to try a different angle on your friend you could point out that depression is a diagnosis, and as long as it is identified and treated adequately the cause of those issues is irrelevant in the immediate term.
The current theory on most clinical disorders in psychiatry is the 'stress-diathesis' approach, which is that a biological tendency towards a condition is set off by 'stress' (something bad happening) in the environment. Chemical inbalances are certainly part of the picture, but not necessary considered the main issue any more.
The DSM IV-TR gives a wide range of symptoms which taken together could constitute a diagnosis of depression. If you wanted to try a different angle on your friend you could point out that depression is a diagnosis, and as long as it is identified and treated adequately the cause of those issues is irrelevant in the immediate term.