If he writes software for manned spacecraft I think that would be terribly stressful: knowing that lives depend on whether or not there are any bugs in your code... I think if I wrote software like that I would be very likely to obsess over it.

And please nobody suggest TDD. TDD has its place but it's not a holy grail and it only tests the tests you think to write. Even if your code coverage is 100% you have certainly not tested every possible input and path if the software is at all complex.