I had no thought about starting to write a song. However, it quickly turned into a song about a poor man whose feet and back hurt because of manual labor. As I continued with it, the story continued to evolve. At some point, the songwriter needs to pin down what the song is about, but it doesn't have to be when you first start writing the song. Dogmatic rules about the writing process don't work any better than dogmatic rules about the structure or content of the finished work.
Creativity is the antithesis of dogma.