There have been lots of talk about the benefits of test driven development, but there is one element that I think is interesting that I not have seen a reference to - the psychological process.
Pretend that we dive into the head of a programmer (e g me) and see what happens during testing. Let us firest have a look at "classical" post-fact tests and then at the TDD way of writing the test before the code.
If I write some functional code and consider myself finished I will without doubt feel immensely proud of myself - in other words my ego is on a high note. Then I sit down to test it, and let's see what can happen: either I find bugs and flaws, or I don't. If I do not find any bugs, I do not get any extra credits in my own books - I have been exactly as good as I thought I was (but I might feel that I have wasted some time). If I do find bugs I get a slap in the face - I have not been as good as I though I was (and have committed hubris, which is a severe sin on the list of things considered as sins in Sweden). So, even if testing will improve the general level of the result, the activity of testing (per se) will never give an upside, can at best have a zero outcome, and at worse a big minus. Depressing.
So, how do I treat activities that give me no upside, but possible downsides? Naturally, I tend to avoid them, and over time minimize them.
So, I tend to shy away from testing and writing tests; even though I am firmly convinced of their necessity. Depressing.
Driving the development through tests breaks that negative feedback loop. If I write the test before the code, naturally it will fail: I have no other expectation, and the red bar is not a failure. Later on, I write some code, which make the test go green, and I have actually finished on a high note. Great!
Well, actually I have not finished as any TDD-savvy would quickly point out. I have not refactored away duplication and other code smells yet. But doing so will satisfy another part of my ego, the code aesthetics lover that resides inside me. So, satisfying that part of my ego will end the job on a yet higher not.
OK, to be brutally honest: are there no part of TDDs "red-green-refactor" that have the same "can only go wrong" drawback as traditional test-last? Well, there actually are: the refactoring. At best it says: you did not mess up, at worse: you think you where so smart but you failed. So, how come I do not tend to neglect refactoring?
Well, I have already given the answer: I love refactoring, if nothing else for the pure love of nicely crafted code. But also, the refactorings are not only the last cleanup of one part of the job. They are most often also the stepping-stone from which I launch into the next part of functionality to write - so I get immediate feedback from the refactoring as well.
I think writing tests in general, and unit tests in particular are really important. Considering how many that agree, and how seldom it is practiced, we must look for explanations: ignorance, arrogance or malice do not suffice. To see how we experience "writing tests" has helped me to shed some light over the mystery.
Whatever, I think one of the drives for me to do TDD is that is make all parts of the development cycle to be fun.