Most classical song composers start with the words first, be it the libretto for an opera or a poem they wish to set to music. A libretto tells a story, and stories involve emotions. Mozart, Puccini, Verdi, the all started with the libretto first. Broadway musical composers usually start with "the book." A well constructed poem invariably has a strong emotional content. Music can be, and almost always is, constructed to express emotions. When written as a setting for a poem (Schubert did a lot of this), the melody can be tailor-made to the emotional content, underlining and enhancing it. I don't see how this is possible when you start with the tune. If you do it that way, rather than the tune being tailor-made, it's rather like picking a suit off the rack and trying to find someone it will fit. I know some people do it that way, but it seems like making the project sort of "catch as catch can." Sometimes it works, sometimes it doesn't. Really good melodies are usually made up from the natural speech rhythms and inflections of the words, which means the words really need to come first.I don't write songs myself, but I know that's the way classical composers do it, and they rarely fail to produce a quality product. And Woody Guthrie always knew what he wanted the song to be about, had a few lines in mind, picked a tune (usually traditional) that seemed to fit, then cobbled the tune to make it work. It looked like he was doing both together, but the words actually came first.
Don Firth