They cannot focus on one thing quite like the average person. Essentially, their stream of ideas is always running — the tap does not shut off — and, as a result, creative people show schizophrenic, borderline manic-depressive tendencies.
There’s quite a bit of sensationalist bullshit in here about writing “resulting in” depression (if anything, it’s probably the other way around) and the old, rusty saw about writing being “narcissistic and lonely.” Also, based on a few of the more spectacularly terrible examples of humanity the profession has put up, the writer concludes that writers are often terrible people. (No mention of the wealthiest Brit ever to have given up the majority of her fortune to charity, of course, or intensely humane individuals such as Terry Pratchett or Douglas Adams.)
But if you disregard that, I find the core issue of neurological similarities between creativity compared to mental illness to be fascinating. And the article links to a number of better-written and less muckraking discussions on the subject.
The Neurological Similarities Between Successful Writers And The Mentally Ill