People often think small and set goals that they think are "realistic". In other words more within their capabilities. This idea of what they think they are capable of comes from a fear of failure or fear of looking like a failure.
The reason people often don’t think big and go after their big dream, their true heart’s desire is to limit the potential hurt and damage they believe is possible if they do not succeed in hitting their target.
This is all an illusion by the way. There is really nothing to be afraid of.
Enjoy!

From Longing to Living:
All in a Life's Work




Bill O'Leary
Derek Sivers