How Do Serotonin Inhibitors Decrease Pain?

Does anyone know how SSRI's decrease pain?  I have read that sometimes they do and sometimes they don't.  But for me they did.  However, the side effects are so bad that I cannot take them.  If I could figure out the pain reducing mechanism of SSRI's, perhaps I could figure out another thing to try.

I don't know if the SSRI"s reduce pain because of the increased serotonin or something else.  I have tried to increase serotonin naturally and it didn't help.  I have also read recently that increased serotonin in the prostate is a cancer marker, so that leaves me uneasy about that route.