Success in the gym is determined by genetics, knowledge, and drive. I think of it as an equation where each factor has an effect on the other factors. Success is not the sum of the parts, but instead, a constraint of factors.
((genetics * a) * (knowledge * b) * (psychology * c)) = x
Genetics is a an individually fixed value and will never change unless in the near future, scientist learn how to re-encode a person's genetic structure.
Knowledge for non-professionals varies greatly and can be the single variable that trashes your entire training program. You can have good genetics with a strong drive to success, but then stop eating because you think it will lean you out causing issues like over training, muscle loss, and injury.
Psychology encompasses drive, but also ego. You can have great intensity, but allow your ego to fuck your training up trying to lift more than you should which can easily stop your gains dead in their tracks. Also, dealing with your problems in your life will allow for a larger reservoir of mental energy needed for focus. At least, this is true for me. A depression will wreck my training, so I have to stay on top of my psychological state.