From personal experience, I feel reduced amounts of gratification from doing mundane things. Mental stimulation is reduced, therefore I'm driven, in some cases like a motor, to find more stimulation. Activity in the brain , in the form of thoughts, is one way to increase stimulation. Stimulant drugs satisfies this need for stimulation, so we don't need to keep on thinking about stuff, like the meaning of life. My theory is that this need to race through thoughts is the mechanism for bad short term memory. Running through more thoughts means old thoughts are forgotten.

There are also different types of Adhd by the way. There's inattentive and hyperactive. My explanation probably fits inattentive ADHD since that's where my personal experience lies.