From "Into the Heart: The Stories Behind Every U2 Songs", by Niall Stokes:
"As singles go, this was a song with a big theme - or series of themes - which were only sketchily executed. A guy Bono knew - "he was an acquaintance of a friend of mine, Sean d'Angelo" - had tried to commit suicide. "In fact I went up to the hospital with Sean to see this guy..."(Bono said)....
The suicide attempt played in Bono's imagination and emerged in 'A Day Without Me', with the protagonist looking back at the world he has "left behind" from the perspective of the grave - or more likely a vantage point somewhere above the graveyard, as he watches the funeral and takes note of those who haven't shown up. "I was fascinated by the thought: would it make any difference if you did commit suicide?" Bono recalls.
Typically, that is just an undercurrent. The song also touches on the theme of insanity, and on the caollapse of the self, reflecting another common teenage insecurtiy."
So, everybody is right.