Tenshi makes great points.
It is not only important to consider the effects -during- an experience, but also the aftereffects. MDMA might push a person to be in a good mood (an incredible understatement, really) it will be so much greater if the person is already positive/in a good mood. My first experience with rolling actually ended up with me not rolling at all.
I had taken a pill with a friend, and informed another friend because I was so excited that I was finally trying something I'd been so interested in. Turns out, this other friend wasn't excited for me at all. He drove to my house and started arguing and yelling me with me. Kept me from ever getting "up" so I never felt the high because he kept me in a terrible mood, even though my friend who had also taken a pill was rolling face. So, I'd definitely say that mindset has a large effect, too.