Dopamine is certainly a factor, but low serotonin is at greater distinction. The dopamine cycle exists to regulate task completion and is associated with reward stimulus. Serotonin is associated with mood regulation. In manically depressed persons and persons with advanced ADHD serotonin production is extremely low compared to normal. The primary compensating behavior then is seeking external mood stimulators. For the person in my family with ADHD their drug of choice is anything with large amounts of red dye 40.

Although persons with ADHD have lesser DRD2 and possibly lesser DRD4 receptors using prescribed medication to raise serotonin production reduces the urge to hyper stimulate and thus drives behavior towards a more normal homeostasis, which then also solves for the forgetfulness.

Dopamine is critical because it is what signals thallamic and striatal signals to the prefrontal cortex which is where the executive function deficits originate.