Reebok is absolutely correct. Ousen is best suited to be a general - he is not the sort of person who can rule, he's not the sort of person who can build up a nation, he can only care for his forces, who have more than just his military goals in their lives. He is a great destroyer, and an incredibly poor father. He might ignore Sei because he's young and inexperienced, but Sei's already proven to be the ruler that Ousen never could be.
Unfortunately, Ousen's one of those people who are so assured that everything they do is right, that any proof or advice to the contrary is only met with scorn and derision.