This is way more complicated than lack of a single thing. It's mostly a mutation influenced by a number of genes, causing changes in the frontal lobe, changing a number of pathways. And motivation isn't even a required symptom. It's also under diagnosed at the same time as over diagnosed, so just mentioning one side of it is not great.

I mean, just give the Wikipedia a go to learn about the scope: https://en.wikipedia.org/wiki/Attention_deficit_hyperactivit...