Dopamine is often referred to as "pleasure"-transmitter, but it's function in certain parts of the brain is better thought of as "regulating expectation", therefore influencing drive & motivation. It makes it possible to work on stuff you don't particularly enjoy by focussing on an expected long-term reward. This works by enabling your conscious thoughts to silence other, more excitatory and emotional parts of your brain via a feedback-loop. [2]
People with ADHS have trouble concentrating because this feedback-loop doesn't work well when they're not moving. Sitting still, they are constantly being flooded with inappropriate and negative emotions. They calm down when taking stimulants like Ritaline, because the "calming"-function of the brain starts working properly.
Bottom-line: Cannabis has many good effects, but it definitely blunts motivation. [3]
[0] rule of thumb: more than once weekly
[1] http://www.pnas.org/content/111/30/E3149.abstract