trust me, I write instrumental music constantly, and one of my more famous songs.. and by famous, I mean at least 5 people on the planet know what it is.. lol. but one of my more famous songs is a very sad song, using the chords: Dm, F major, Am, and Em. I wrote this song back when I knew next to no theory, which becomes obvious when the solo comes around and I go way off beat... but that's not the point.

with alot of things I write, I try to figure it out using theory, saying, ok, this chord fits in the same key as this chord, so they should go together... and they come out ok at best. other times, there is no theory going through my head, I am only playing chords that I feel are expressing my current emotion, I make chord changes when I feel like it, and my strum/pick pattern is as fast or slow as I want it to be... these usually end up being my best songs that I write. so no, there doesn't always have to be theory going through a songwriter's mind, there can be just pure simple emotion... and I mean, yeah, after the song is written you can find all the theory in it, but that doesn't mean the songwriter intended for it to be that way.