I think its because your body fights THC and tries to get rid of it. I am not sure but from what I understand euphoria and some sensations are not the result of the chemical by itself, but rather our body fighting it off. If not, then why do people often feel tired when the high wears off? I know a kid who has never been high for more than 1 minute. He passes out from bud almost instantly (not sure how, since it doesn't let you have Stage 4 sleep...).