Mental stability and having a strong character is essential for any endeavour or concern. This is true because you may fail, stumble and fall at some point.
You therefore need courage and emotional stamina to pick your self up and move on.
The ladder of success and excellence is never easy. It is full of difficulties and challenges.
“A man of character finds special attractiveness in difficulty; since it is only by coming to grips with difficulty that he can realize his potentials” - Charles de Gaulle
David saw Goliath as an opportunity to excel (and actualize his dreams) and he refused to be discouraged by his eldest brother's rebuke (1 Sam 17:28) or become fearful due to the timidity and fright of the soldiers.