Michael Pollan, in Botany of Desire points out that forgetfullness is the key point of a marijuana high..

it is human natare to have vast portions of our brain on autopilot.. we learn something, and then if we need it, its saved.. but the experience fades.. (like in economic- no matter how good something.. we acquire it, and become satiated..)

with marijuana, everything is new again.. every experience takes on the fear, joy, excitement of a new experience... it is as if we have forgotten everything.. so the twill weave of our jeans becomes facinating.. and thing we have grown blase about.. are experienced, as if for the first time..

we have a child like wonder.. think of your favorite anything.. and think if you could experience it, as you did the first time.. get a marijuana high and you can..

and yes, long term, it can interfer with our abilities to form memories.. but every once in a while it nice to see the world through brand new eyes!
(not that i do it..or maybe i just can't remember doing it. )