Is Five Days Darkness hatred irrational? FDD likes the Solars. They remind him of his brother/thing he is the Shadow of, the UCS. The Dragonblooded killed them all. Also, they expect him to listen to the Immaculate Order, which is why it makes sense that he went out and learned the Styles of the Immaculate Order.
Or wait, is it tied up to his Hatred of the Elemental Courts, instead? I don't remember.