Everything we do is by choice. Everything except death. We choose to obey laws, to see life through black or rose-tinted glasses, to get up and do what we love or sometimes what we hate. Life is an endless stream of choices. Some are made consciously, some unconsciously, some compulsively, but all are made nonetheless. For most of us, it's the latter two that have the greatest impact, for they accumulate over time and end up shaping us, often into something we hate.
You can always choose not to choose; you can always choose to forget and let go, but the choice of inaction is also an action, and like any action it has consequences, and they will always be there, waiting for you at the other end of your complacency.
Discipline is nothing more than making your unconscious choices conscious and challenging them before moving forward. It's freedom because it means you choose not to be enslaved by your primal desires and instead strive for more.