The Birth of a Song

You know, I've seen bands and artists do this in different ways, but I do think that the most common way a song ends up being developed is by having strong music first.  In my own experience at least, a song usually starts with music, created on the guitar or piano (those are my two main instruments), and the vocals sort of come to exist from there.  Most times, I won't even know what the lyrics are going to be about, but depending on whatever melody I start humming along with the music, a simple sentence or phrase will come out of my mouth and not make much sense.  But that simple little phrase or sentence could be the skeleton of the song, which the rest of the lyrics end up being built around.  It's very interesting how music comes to be born.  Also, I tend to be a really dark and emotional songwriter, so usually when I pick up my guitar or start playing on the piano, I'm already in some kind of mode.  It's sort of like a possessed state that I can't snap out of.  So, what happens is I just start making music on an instrument and from there, depending on whatever I'm going through in life, the words will just sort of flow out.  The chorus of the song usually is just some repetitive line that sticks in my head and touches my soul for whatever reason.  It's usually just a few words.  I know that's the description of almost any rock song in existence, but I just figured I'd share my point of view as a songwriter.  I'd love to hear other songwriters and bands' perspectives and how they go about writing songs themselves.