"I responded, and the image was made."

   The photographer Roy DeCarava, in an interview many years after the fact, described a photograph he took of men coming out of a memorial service in Harlem for the children killed in a church in Birmingham, Alabama.  The “men were coming out of the church with faces so serious and so intense that I responded, and the image was made.”
   This is the answer, or at least a good part of the answer, to the person who thinks, of any particular shot, "Well, if I were there at that time, and I had that camera, I could have taken that picture.  I could have lifted the camera and pressed the button."  But you could not.  Unless you first saw what was there.  And you had it in you to respond.  Not just "take" the picture.  But respond.