If we knew how to predict people's reactions all the time, there would never be any strife. (Unless someone was just being a jerk.) Relevent to this post, I remember reading something from a Disney person talking about people telling them to keep making box office hits like Lion King. The remark was that if Disney knew exactly what caused a movie to be a box office hit, they'd always make them!

So many aspects of life are trial and error. No getting around it.