I think at the bottom of every human action and reaction, there is an equation. The equation that he/she shares with someone or something. Catalysts may or may not be present. A person's behavior, ideas, capabilities and emotions are shaped by this equation. It does not mean that a person has no control over his life but his personality is likely to be shaped by these equations.
When i first heard it, I was mesmerized by the quip that life is 10% what happens to you and 90% how you react to it and I still am, but it is deceptive to think that the story of a person's life is based on his reactions alone. It is a sequence of equations. You never know what's going to happen at the end because one or more components of these equations are unknown. You are one part of a given equation and the presence of other entities in it changes the output of the equation and contributes to what you become.
Victor Frankl was an amazing human being, but he would have probably never written his book or identified his immense strength if he hadn't been subjected to torture.