Robert Harris warns against the desire of starting it all over:
We’re programmers. Programmers are, in their hearts, architects, and the first thing they want to do when they get to a site is to bulldoze the place flat and build something grand…It’s important to remember that when you start from scratch there is absolutely no reason to believe that you are going to do a better job than you did the first time.JOEL SPOLSKY IN THINGS YOU SHOULD NEVER DO, PART 1
There is a fleeting moment in every software project when it is absolutely perfect. It is the time between clicking “New” and “Save” in your code editor. In that brief interval, limitless potential and beauty. In every moment that follows, compromise and doubt (but working software, too!).
There are a few threads to unravel here.
First, Chesterton’s fence: if you don’t know why a thing is there, you are probably not the right person to decide to remove it. If you understand why the code is there and exactly what it is doing, then you become qualified to decide what, if anything, needs to be changed.
Second, ego: I’m a great developer. The best developer I know. Heck, maybe the best developer in the world. Therefore, if I don’t immediately understand code, it must be because that code is bad. Most of us don’t think explicitly in these terms but we still end up in the conclusion of, “if I don’t immediately understand the code, it is bad.” Or even worse, “If the code does not work exactly the way I would have it work, it is bad.”
Third, unstated/misunderstood business requirements. Code often starts to get nasty because the business requirements changed on the original designers or there was a process of business evolution. If business requirements are still evolving, what makes you think you’re going to write code that won’t be just as ugly? If business requirements are not still evolving and you really understand the code, you have a chance. But that leads me to the next bit.
Fourth, the value of reformation. Refactoring is a common path for code reformation. Having lots of tests increases the safety net we have for reformation, as those tests are likely to catch some of the dumb mistakes we make and hopefully suss out some of the worst things.