"Can't", to me, implies fearfulness, lack of courage, and lack of intelligence and/or imagination. Can't is the opposite of Can. Can't then would mean that the ability is present, but that fear has taken control and blocked the persons will. This is weakness at it's core. You must have power over your mind and you willfulness. If you are too weak to override your own mind's fears then who are you to even try to present yourself as a powerful, person able to accomplish a perceived difficult or impossible task.
There is a solution to any and every problem, a person just needs the courage, determination and imagination to solve problems. Cowards and liars allow the word "can't" into their vocabulary.