Serotonin seems to be only one of a bunch of various factors that are thought to have relationships to mood/susceptibility to depression. As I understand their function, the SSRIs don't add extra serotonin as such, but leave more of it around for the receptors to pick up. Correspondingly, I'd guess some of what happens would also rely on the efficiency/tuning of those receptors. Dopamine levels also appear to play a part & who knows what other interactions are going on. [This is where somebody who's studied brain chemistry needs to magically appear & explain all]

Disclaimer: All the above comes merely from my own diggings for information & may be utterly wrong/misinterpreted. I guess my main point can really be boiled down to this: The brain & it's workings appear to be really complicated & it I think that we're still working out what really causes what.