...but reduced serotonin levels are linked to depression. The entire point of SSRIs is to keep your serotonin levels higher and thus improve synaptic signalling. Wouldn't reducing your serotonin production be bad?
That's a point of debate, actually. SSRIs in the long term appear to actually work by reducing serotonin in the brain tissues.
Serotonin is really more of a torpor hormone that depresses the whole body than a "happy" neurotransmitter. It's associated with hibernation, for example. The "turkey causes sleepiness" meme is about tryptophan in the meat boosting serotonin levels.
Edit: Here's a link to a cross-sectional study showing a cause-effect relationship between smoking and depression. http://bjp.rcpsych.org/content/197/5/413.full