To study this, the researchers used "radio-labelled dopamine" ([18F]-DOPA) in a group of cannabis smokers and a group of controls. Dopamine is the chemical in the brain that controls pleasure and motivation (read here). The study found that cannabis smokers (3xweek) had decreased levels of dopamine. Since these levels are decreased, researchers think this will contribute to the lack of "pleasure" of the cannabis smoker and therefore, the smoker will be make less effort to obtain something. And that is why cannabis can make you lazy! This is probably what inspired this song:
The study can be found here.
Bloomfield MA, Morgan CJ, Egerton A, Kapur S, Curran HV, & Howes OD (2013). Dopaminergic Function in Cannabis Users and Its Relationship to Cannabis-Induced Psychotic Symptoms. Biological psychiatry PMID: 23820822