An alternative explanation is "No man ever steps in the same river twice, for it's not the same river and he's not the same man."

Certainly holds true for me: the experience, it turns out, depends more on where I am in that stage of life than the substance in question. Cannabis is radically different for me now (at 30-something and after several intense psychedelic experiences) than at 19 or 20 when I first tried it.

And...I'm not assuming you're a man :)