Everyone forgets things, but amnesia is still a distinct condition. Everyone has times when they're less motivated, but it isn't depression. Everyone bleeds when cut, but that's not the same as hemophilia. Everyone has headaches, but that's not the same as being prone to migraines.
In all of these cases, the key practical difference is in the extent and impact of the issue. Is the trouble focusing on boring tasks something that's annoying but manageable, or does it get in the way of finishing school or holding down a job?
Physical issues often have a known mechanism that exacerbates the problem. We don't have a great understanding of mechanisms behind most mental health issues at the moment, so both diagnosis and treatment are based on symptoms. I understand there's research showing that ADHD is highly heritable and related to specific structural differences in the brain; I'm not sure how strong the evidence is, but it definitely points to some mechanism behind the condition.
In all of these cases, the key practical difference is in the extent and impact of the issue. Is the trouble focusing on boring tasks something that's annoying but manageable, or does it get in the way of finishing school or holding down a job?
Physical issues often have a known mechanism that exacerbates the problem. We don't have a great understanding of mechanisms behind most mental health issues at the moment, so both diagnosis and treatment are based on symptoms. I understand there's research showing that ADHD is highly heritable and related to specific structural differences in the brain; I'm not sure how strong the evidence is, but it definitely points to some mechanism behind the condition.