The thing about discipline, is that it endures failure; it can take setbacks, let down, and disappointment. Discipline endures all the drama of growth. When you have discipline, you understand that challenge is inherent to expansion and accept it as part of the process.
We don't expand as a person or attain lofty goals simply by wanting it to be so. It takes desire and discipline to get there. Desire is the fuel that pushes you forward, but discipline leads you to learn from the bumps in the road. That in itself is WHY discipline gets you there: you learn from the setbacks and become better for them!
Desire forms goals, but discipline accomplishes them. Nothing extraordinary was achieved with desire alone.