I remember watching a program on TV years ago about ecstacy, and it basically said the drug causes your body to release something in your brain (forgot the name now, someone in here will know) which causes the euphoric feeling. The issue, it said, was that there was only a limited supply of this stuff in your bain and once it runs out, it's gone. This led them to conclude it could lead to severe depression later in life as you've basically ran out of the happy stuff.
Seretonin and yes, it takes between 30-90 days to restore it to the level it was at before.