Roosevelt isn't merely saying that willpower opens doors—he's suggesting that discipline is almost a form of freedom rather than constraint. Notice his careful word "most" rather than "anything"; he's being honest about limitations while refusing despair. A surgeon spending years perfecting her craft through grueling repetition discovers that the daily drudgery of practice eventually becomes indistinguishable from the joy of performing—the discipline remakes her very capacity for satisfaction. This matters because it flips the common complaint that self-denial postpones happiness; instead, genuine discipline is what makes happiness possible in the first place.