MDMA causes your brain to dump a load of seratonin at one time.
SSRIs regulate the flow of seratonin in your brain. If you are taking an SSRI, it is impossible for MDMA to do its job, as your seratonin is already being regulated by the SSRI. The whole point of the SSRI is to maintain a constant level of seratonin.....which is the exact opposite of what MDMA is meant to do.
You may get very mild effects from MDMA if you are on an SSRI, but you will never get the full effect because the two drugs are working against one another.
Seratonin is your mood regulator. Its what makes you "happy". If you are suffering from depression, coming off your meds and then taking MDMA is one of the worst things you can do. As I already stated, MDMA causes a large amount of your seratonin to be released at once. When you are finished rolling, you are left with a huge seratonin deficit. It takes your body several days to replenish that seratonin and during those several days, your depression will be aggravated. Most people, even those who don't suffer from depression, experience a "blue" period, usually two and three days post roll.
As for 5-HTP: It is a pre-cursor to seratonin that your body will readily accept. Thus, tricking your brain into producing an excess of seratonin in a much shorter period of time than your brain would do on its own. Taking 5-HTP daily is highly recommended to those who roll and, as a side note, 5-HTP has also been shown to help all people suffering from depression, not just those who roll... negating the need for an SSRI.
Hope that answers all your questions on SSRIs and MDMA. If not, please ask away!!!