Fixing fresh bugs is easier than fixing old ones. It’s usually fairly quick to find a bug in code you just wrote. When it turns up you often know what’s wrong before you even look at the source, because you were already worrying about it subconsciously. Fixing a bug in something you wrote six months ago (the average case if you release once a year) is a lot more work. And since you don’t understand the code as well, you’re more likely to fix it in an ugly way, or even introduce more bugs.
