I imagine their backgrounds were a huge determining factor:
-- Hal's father died pushing the boundaries and sometimes the boundaries push back. Hal spends his life fighting the thing that took his dad by continuing to push the boundaries.
-- Bruce's parents were killed by a man with a gun when he was a powerless boy. Bruce spends his life fighting guys with guns with anything but guns and while he uses weapons, he has honed his mind and body to be the ultimate weapon.
-- Hal had his mother and brothers as a support system. He then joined the military, so he understands working with groups and devotion to a higher cause. However, he maintained his individualism and always challenges authority.
-- Bruce traveled the world, and while he had a series of teachers, was basically alone committing himself to a personal quest. However, he keeps trying to cobble together a family with Alfred and his Robins/associates.
-- Hal works in space all the time, thriving in new situations and dealing with the unexpected. After losing his dad, he felt a need to get out. He's the ultimate extrovert.
-- Bruce works in his cave underground. Batman has become synonymous with Gotham. He knows every little inch of that place and thrives when he can plan ahead. After the city took his parents, he felt the need to make his home safe. He's the ultimate introvert.