tl;dr: "The scientists found that the more the marijuana users reported consuming, the greater the abnormalities in the nucleus accumbens and amygdala. The shape and density of both of these regions also differed between marijuana users and non-users."

Nucleus accumbens: responsible for pleasure & reinforcement, maternal behavior. Direct stimulation has been shown to successfully decrease the effects of depression.

Amygdala: Responsible for emotional learning, memories of emotional experiences, memory consolidation, and emotional regulation.

All sources Wikipedia. I am not a neuroscientist, just trying to get a better understanding.