This is not what you will find to be teh case, where did you get teh scientic data that serotonin levels is the cause.
The issue is that serotonin is one neurotransmitter involved in mood regulation (which also involves the systems of arousal, pleasure and circadian rhythms)
The main point is that the serotonin system is widespread in both the CNS and PNS and many of its functions are regulatory. The exact theories about why SSRIs have an effect has changed a lot since I went to college and got my degree back in '86, so I really would have to do a bunch of reading.
The main thing is that if you have a system that regulates other systems and that system gets over active then it causes the homeostasis in other systems to go off kilter. By adding the SSRI and consequent effects of more serotonin, in some people the system comes closed to regulation.
This is just some recent citations for the
serotonergic theories of depression
I wish I could read this one by Beck:
Biological Underpinnings of the Cognitive Model of Depression: A Prototype for Psychoanalytic Research
but this one is great
Monoamine Theories of Depression: Historical Impact on Biomedical Research
"The article argues that the impact of monoamine theories is best explained by the ability of researchers, governmental agencies, and pharmaceutical companies to invoke theories that advance various projects and agendas."
However that is a sidebar and answer the original point of teh OP, other theories are being investigated now.
Much more interesting are the citations for
serotonergic mechanism of depression
This would be a great read!
Integrative physiology of depression and antidepressant drug action:
Implications for serotonergic mechanisms of action and novel therapeutic strategies for treatment of depression
Which I recommend peeking at as it hints at the much more complex role of the serotonin systems. (I say hint because you can only see snippets)