Many things come to my mind when I think about what songwriting is , many say it’s all about the lyrics, some say it’s all about the vocal melodies but at the end of the day I think songwriting it’s about more than that. 

When I compose a song I keep in mind that the end product must be very well balanced. 

So in my opinion, all components of a song: structure, dynamic, textures, sounds, lyrics, vocal lines, vocalist are equally responsible for a good or a less good tune.