I think Dual Woofers said it: The pride factor is very important for a DIYer, although this means that the project (or mod, for that matter...) has been successful. Now, "successful" is a very tricky word indeed. I have met DIYers who have built electrically correct i.e. working, noiseless gear which sounded rather awful, for some reason. They, then, tried to persuade themselves for some time at least, that this was the best thing since white bread! I have also met many others who, liking a certain vintage component, modified it so much with audiophile components, that the poor thing lost its soul and became something else altogether (at great cost...)!
To cut a long story short, DIY is something for people who are curious, like to do things with their hands and - last but not least - are psychologically ready to take the risk of failure and scrap the thing altogether. Or make a cheap first attempt, before seriously indulging in the construction of a project. That's exactly what I did with a tubed phono stage I built lately (see attachment. For more, go to: http://www.audiocircle.com/circles/viewtopic.php?t=10530).
Finally, a good DIY project is carefully planned and executed over a rather extended period of time. This, IMHO, is a big part of the pleasure also permitting to avoid mistakes and also to spread the total cost into smaller increments. At the end, an - eventually - expensive - project seems less so. Especially if it provides the joy it is meant to!
Just my 2c...