Association is not proof of cause.

Disassociated anecdotal evidence is not empirical.

Dopamine levels may be decreased because the ingested cannabinoids reduce the need to produce it. Dopamine levels may be reduced from a number of psychological states, that are commonly self medicated with cannabis, as the current psychological treatment model is largely ineffective.

Empirical studies do not support the cannabis amotivational syndrome concept, I'd expect this point to be made clearly by a periodical begging to be regarded as scientific.