To offer a slightly different perspective, I would put forward the idea that your song doesn't *have* to be about anything.
Sometimes I'll have a very definite idea of what I want to write about, and sometimes that makes the lyric writing process extremely laborious. When I decided to write a song about 9/11 I though it would be quite straightforward because there's so much to say. Turns out it took 3 and a half months to write the bastard thing! Probably won't tackle that again.
But some of my songs begin just singing 'shapes' over a chord progression (which is how I tend to write my melodies). Sometimes the shapes feel so right, that I begin trying to find a first line or two that fit those shapes - and that dictates the lyrical direction without making a conscious decision about themes.
Those who have heard my songs will also know that I'm not afraid to be a bit abstract. Maybe too strong a word, but I don't mind suggesting a mood, image or theme to the listener without making it obvious. I find this approach can open your song up in new ways, and lead you toward different lyrical material than you might otherwise write.
Sorry if this doesn't really answer your original post, I just thought it was an interesting thread, and I had something to say!