Often it happens with us that whenever we start writing code, we just keep on writing without focusing on the maintenance of the script. This may happen because we are in a hurry to complete the project. But eventually, any haphazard code is difficult to maintain and the code written by one developer would be difficult to manage for another developer.
For example, when you do the coding yourself, you may think that you have done a great job by delivering everything on time, but when you revisit your code after six months to do some modification, it often happens that you forget what you had written and what logic had been applied.
So being a good developer, your focus should not only be on the delivery of value added analysis to your end user, but also on maintenance of the script. Unless you handle your script well, you cannot truly call yourself a good developer...