Here's where we stop saying "genius and insanity" loosely. There are connections betweencreativity and mental disorders.

More specifically, obsessively thinking about things is connected to depression – but it's alsocorrelated with creativity:

Because rumination may allow an idea to stay in one's conscious longer and indecision may result in more time on a given task, it was expected that these two cognitive processes may predict creativity.

This rumination/perseverence connection can be a double edged sword for creative people:

"Successful writers are like prizefighters who keep on getting hit but won't go down," Andreasen says. "They'll stick with it until it's right. And that seems to be what the mood disorders help with." While Andreasen acknowledges the terrible burden of mental illness— she quotes Robert Lowell on depression not being a "gift of the Muse" and describes his reliance on lithium to escape the pain— she argues that, at least in its milder forms, the disorder benefits many artists due to the perseverance it makes possible. "Unfortunately, this type of thinking is often inseparable from the suffering," Andreasen says. "If you're at the cutting edge, then you're going to bleed." [Imagine: How Creativity Works]

Steve Jobs used to talk about products that were "insanely great".

That makes more sense in this context.