One characteristic of excellence in software development is the capacity to make optimal decisions every day as new information becomes available. You don’t want to be over committed to an artificial version of reality, which is what most plans and schedules really are. Change is pregnant with opportunity. If you are too committed to some course, that might make it difficult for you to adapt to the rapidly changing circumstances in which you find yourself. You might miss many of the most rewarding opportunities that present themselves. You’ll perceive them as obstacles, as problems with the schedule or the plan, rather than as the creative opportunity they are.
… Like a child, each project is unique, with a life and personality of its own. You must validate the project’s unique identity as you encounter signs of it and supply the particular project adequately for its journey. You have to relate to each project on its own terms, not according to some cookie-cutter process you’ve deemed up or "learned".
And I will add: "or a process that’s been handed down to you.