Nothing like being in the zone when you're going after a goal. Vision is clear, focus is sharp, and you live and breath your pursuit. What was once a challenge becomes comfortable and easy.
Which is why distractions are so frustrating. You want to stay focused and not be bothered. Some distractions are necessary and must be dealt with, but some are just the forces of evil trying to get in your way.
That is where discipline and the ability to know the difference comes into play. And so many people lack discipline - they simply become distracted and never return to the pursuit of their dream. Once out of the zone, they rarely return. Small things distract them from the future.
Discipline is choosing what you want for your future over what you want now.