Well we all seem to be getting somewhere. I think everyone has learned something. Personally I have learned a lot. One thing worth considering is that lyrics can be a melody in themselves.
Our natural speech contains a lot of intonation and spacing of words. A tune in itself if you like. It is sometimes easier fitting a melody to lyrics than lyrics to a melody. Some lyrics almost form ther own tune as you speak.
Now I know that most folk here are country orientated whilst I am classic rock however the "rules" are very similar.
I am a big fan of the band YES. Now to illustrate a point about lyrics carrying its own tune. They took lyrics to extremes not just music. Most of the Yessongs are quite complicated pieces. The lyrics are just a collection of haphazard words. They do not tell a story and mostly do not even make sense. They just sound good. They chose words just for their sounds to compliment the melody and each other. Weird but it works. Suggest listening to Close to the Edge by Yes.

http://www.soundclick.com/bands/songInfo.cfm?bandID=687727&songID=5238055