...could you talk about your relationship issues without the guitar?' (see the Bizarro comic here)

The cartoonist begs the question: how does self-expression migrate from self-indulgence to something we might call art?

My guess is that some combination of awareness, perseverance and genius is involved - but I'm not sure about the genius.  Maybe just the first two, plus luck, will shine the light on the next Bruce Springsteen or Edna St. Vincent Millay.  With genius, you get what you get - but awareness and perseverance take work.

Lack of awareness lets the singer-songwriter foist her relationship issues on the hapless listener.  And don't be too quick to let the activist folksinger off the self-indulgence hook.  Songs about union rights and West Bengali tea workers do not ipso facto extract the ego from the brew.