Disclaimer: Not mine. Just borrowing.
His brothers were men of bold passions. Robert's passion for beautiful women was sung about from the Arbor to the Wall, and Renly's passion for beautiful men was whispered about for that same distance. His brothers reveled in life with no thought for the consequences of their actions.
Stannis' passions were law, justice, right. Quieter passions, to be sure, but no less intense. He had learned as a boy that the world was cruel and chaotic, that a man could have his home in sight and still be taken by the sea.
Someone had to set the world to order.