You have to be willing to focus your attention down to a very narrow area to the utter exclusion of everything else - in a world of incredibly interesting things.

To the extent that someone is a curious generalist they are not 'cut out' for a PhD. I knew I wasn't. Major reason I went for a MS, then stopped.

As the Brits say: "Horses for courses". The trick is in knowing which kind you are beforehand.