I don't think that serotonin level alone is necessarily going to be the only factor in whether or not (or to what degree) you'd feel the effects of depression - I'm not even sure that there's a direct, non-invasive way to measure it beyond the brain blood barrier. A person's biological makeup is going to have an impact on susceptibility to depression - the physical process of serotonin transmission may be more effective in some people than others & other differences in brain chemistry would also probably produce a different result in one person than another. Your learned thought processes could additionally make one person react differently than other for the same given set of stimuli & serotonin levels. Add in the human bodies capability to self-tune hormone levels & blood chemistry and you have another variable in the mix that could cause variation between different individuals.

[Oh wow - babysteps, was that Dr Delgado reference to this guy? -> http://en.wikipedia.org/wiki/Jos%C3%...riguez_Delgado ]