ADHD is a physical phenomenon. A part of the brain (the prefrontal cortex) That acts like the conductor of an orchestra (controlling the “band” and telling everyone when it’s their turn to play) is affected and therefore, cannot work properly. Imagine if all the musicians played their parts whenever they wanted. It wouldn’t be music, it would be noise. It’s like there’s no leader helping them organize themselves. Impulses take over.
Medication can help slow them down enough to think things through. Maturity, self awareness, and repetitive training can eventually help their functioning also.
Meds are not the solution for everyone though. It depends how severely their lives are affected by this condition, and whether they can tolerate the meds.