A new paper published in Psychiatry Research sheds some light on this phenomenon, or why smoking weed seems to unleash a stream of loose associations. The study looked at a phenomenon called semantic priming, in which the activation of one word allows us to react more quickly to related words. For instance, the word "dog" might lead to decreased reaction times for "wolf," "pet" and "Lassie," but won't alter how quickly we react to "chair". Interestingly, marijuana seems to induce a state of hyper-priming, in which the reach of semantic priming extends outwards to distantly related concepts. As a result, we hear "dog" and think of nouns that, in more sober circumstances, would seem to have nothing in common.
He warns that "you don't want too much hyper-priming, or else everything seems connected; the web of associations becomes a source of delusions." Vaughan Bell has more along those lines.