Is depression caused by life events? If we look at physical illnesses we could put them on a spectrum of 'Happened spontaneously/no choice in the matter' to 'Was clearly created by something that happened in their life'. 

From less causal, to more causal I'd suggest a list like this. 


Genetic disorders.   (Entirely genetic, from birth)
Down syndrome. (Genetic, but age of parents plays a part)
Cancers that happen 'randomly' (eg. breast cancer).  (Random, but some people have genetic disposition)
Common cold. (Not sure where to put this on the spectrum, but caused by a cold, but some people are more susceptible than others). 
Obesity. (Some people have a higher disposition to obesity)
Smoking related illnesses. (Caused by smoking, but some people more susceptible than others). 
HIV.  (Caused by a virus transferred by contaminated blood/unprotected sex)
A broken leg. (Caused by physical trauma)


That is - the illnesses at the top of the spectrum have some kind of genetic cause, or we don't know enough about them to say how/why they occur, whereas the illnesses at the bottom of the spectrum are clearly caused by some life event. 

With a mental illness like schizophrenia - it seems clear that this would fit at the top end of the spectrum - the schizophrenic was schizophrenic from birth, and there was nothing that 'caused it'. 

However, it seems that depression is often the result of life events - stress, addiction, pregnancy, grief etc. 

Is this a valid way of looking at depression? What terminology and research is there about it?
Both schizofrenia and depression are linked to a genetic predisposition and life events. Individuals with no genetic predisposition may become depressed when the trigger is sufficiently intense, while individuals with genetic predisposition may become depressed in the absence of clear life events triggering the depression (Kendler et al, 1995).

Reference
- Kendler et al, Am J Psychiatry (1995); 152(6): 833-42