This naturally occurring chemical is important to many human functions.  However, since it cannot be injected directly, drug companies produce Drugs that prevent the body from absorbing it (uptaking) so there is more available.  The selective prevention of the uptaking has resulted in several classes of drugs that really screw up human beings.  SSRI's and SNRI's can, even when taken under a Doctor's prescription, cause horrendous side effects, while sometimes providing actual relief from certain problems.  The real challenge is how to stop taking the SSRI/SNRI drugs.  

First, read about all the things that Serotonin does in the human body.  Just read past the complicated stuff and catch the parts that make sense to you.