Jack is using psychology to make Ralph seem weak in front of the other boys. Ralph, feeling his power and his influence slipping away, feels he has no choice but to rise to every occasion, especially if Jack seems willing to do things while Ralph refuses. It is this ego-driven impulse that causes Ralph to be more foolhardy, as otherwise it would appear that Jack is more brave. Without that drive, and with the ability to inform the boys rationally, Ralph may have resisted the urge to out-perform Jack, and become a better leader.
