1. Serotonin is a very basic, very old transmitter found in all bilateral animals. Humans have two sites of serotonin-production, one in the body, one in the brain. 90% of serotonin resides in the body.
2. Serotonin in general regulates "activity". It affects hunger, gut-movement, sleep, cell-growth, mood, body-temperature, blood-pressure and many other things. Any drugs changing serotonin-levels also affect these areas, that is why there are so many adverse effects.
3. SSRIs help exactly one group of people: those who have too low levels of serotonin. When their serotonin is boosted, circuits in the brain like the connection between thoughts and emotions start to work properly, hence they get more in touch with themselves.
4. People who have too little serotonin can show the exact same symptoms like people whose serotonin levels way too high (!!). Prescribing SSRIs to those people will worsen their state and may even lead to life-threatening conditions.
5. The level of serotonin can be tested properly by exactly one method: laboratory blood/urine sampling, which costs a couple hundred dollars, but is available. These doctors generally also know about amino-acid therapy, which consists of nutritional supplements which help the body in manufacture the missing transmitter. This can lower the need for medication, but needs proper testing first.
6. People can have proper serotonin levels and still be sad/angry/depressed. The question still is which brain-circuit is malfunctioning. If the problem is the connection between emotions and thought, serotonin helps. If the malfunctioning circuitry concerns attention regulation [0], like with people who have a genetic disposition towards ADHD, then dopamine helps.
7. People can have all their neurotransmitters adjusted to proper levels and still not be perfectly well. We are talking about signalling inside mutable structures here. Changes in signalling affect the structure, changing the structure affects the signalling.
Fixing neurotransmitters makes someone able to use all of his brain. He still has to use it properly though, to get better.