It's based on the fact that you really start to appreciate the test suite once it has saved you from breaking the application a couple of times.

Like when the tests told you that while you tinkered in this part of the code base a totally unrelated and completely separate feature stopped working. Only it wasn't so unrelated after all. And if not for the test no one would have realized there was a new bug in the system until days or weeks later when someone (I bet an end user too) reports that something isn't right.

So after writing tests over a period of time you realize the test suite is your safety net that will catch you when (not if) you fall. This is doubly true when someone is new to a code base, so by writing tests you ensure that neither you nor other people screw up the code.

Anyway, what the guy said on the mailing list was something like this:

You continue writing tests until boredom overcomes fear.

And I think that's very true.

/J