How fast and dangerously I drive is directly proportional to how easily I can kill myself in the vehicle I'm driving. For example: if I'm driving my 93 Buick that goes 0-60 in, oh say... 3 days, then I'm a very safe driver.
If I'm driving a smaller, faster car, my speed goes up. It just sort of happens. I can't wait to see what I'm like on a fast bike.
They drive like that because it's fun. Pure and simple. If I'm going 150 mph and not wearing a helmet and I die, well, at least I won't have to make any more payments on the Ducati I just crashed. (You know, hypothetically)