[Bjartr]

ADHD is mostly understood to be a failure of executive function, which include brain processes that are involved with allocating cognitive resources to tasks. As such you get periods of poor focus from under-allocation of resources. At other times you get periods of hyper-focus from over-allocation of resources. This is also why certain stimulants work for those with ADHD, it stimulates the part of the brain that deals with executive function.

[pekk]

Alternatively, people who have trouble being motivated by something boring are diagnosed with ADHD. But no, let's instead use the explanation which is very complicated and sounds big but is hard to falsify.