Too much serotonin is the issue. Where there is too much action on those receptors serious things can go wrong.

Normally we all have serotonin working for us. There are many types of receptors too, located all over the body. But when you add drugs into the picture, this sensitive system can get overloaded.
And it varies from person to person too. Some people just normally live with high serotonin levels, and those types would be more prone to serotonin syndrome. It is thought for example, psychologically that timid or careful people are high serotonin expressors. Contrasted with high dopamine people who are drawn to excitement and risk taking activities. You won't find many high serotonin types sky diving, for example!
Those with high serotonin systems, often get the worst side effects from SSRI antidepressants.

http://en.wikipedia.org/wiki/Serotonin_syndrome

90% or so of your serotonin is in the peripheral areas of the body. GI tract and platelets mostly.