Depression and other mental illnesses are defined by a set of symptoms. Suggesting one particular cause is being disingenuous. Having said that, the whole notion of "too much" or "too little" of something like serotonin is a gross oversimplification, at best useful as a metaphor to explain things to people at a very high level.

If you want to take it a step further, think about what's causing communication via neurotransmitters to be slowed or sped up (not enough available? not being released? not being picked up by receptor?), the fact that there are multiple receptors for each neurotransmitter which drugs may or may not manipulate, the fact that there are multiple neurotransmitters (which end up affecting different areas of the brain and hence different symptoms, though some symptoms are influenced by multiple neurotransmitters), and that putting all these things together to result in the right balance of communication (not quantity of chemicals) in the brain, it would be a surprise if there weren't multiple different approaches for the "same" problem.