This is semantics, but elevated serotonin levels alone are rarely strongly associated with well being and euphoria, though, and are more often associated with altered perception and cognition. Those latter effects are part of MDMA also, but it's most likely that something else is going on causing the euphoria. Otherwise, SSRI's would generate a similar range of effects, but anyone who has experienced both of those substances knows that they are widely different.

That aside, of course there's a drop off relative to the high, but what I was getting at is that the comedown is not at all like "It's basically like stealing all the potential happiness from the next two days". Plenty of people who don't dose it too high or often, and make sure it's not cut with other stimulants, don't have a severe hangover for multiple days. Often times it's more just like a moderate alcohol hangover that lasts through the morning. There's tons of Erowid stories to show that intense multi-day hangovers are rarely the experience of people with the pure substance who aren't abusing it. The duration and intensity of the comedown is more what I was getting at being (usually) inaccurate.

Still, though, given how common cut ecstasy is in this country, a more intense comedown is sometimes fairly likely due to the factor of impurities. Hence why people should test their product whenever possible.