Love is a powerful emotion, as are hatred, lust and anger. To feel one of these is to be unbalanced, to feel them all is to be swept away.
To feel them all towards one person is more than anyone should have to bear.
Subaru is far beyond unbalanced. Subaru is probably beyond insane, at this point.
Subaru has loved Seishirou for years, and hated him almost as long. The one does not preclude the other. Subaru will never forgive Seishirou for what he has done to him, but he also realizes that the lack of forgiveness is meaningless. Seishirou doesn't care. Subaru has never stopped loving Seishirou, not even as Hokuto was dying. Seishirou doesn't care about that, either.
Subaru is defined by Seishirou's actions, existing only by the Sakurazukamori's will. He doesn't even know why he's still alive, and wouldn't be comforted to know that neither does Seishirou.
Subaru's Wish is a fluid, ever-changing thing; sometimes he Wishes to kill Seishirou, sometimes to be killed by him, frequently he only wants to be loved by him. Very occasionally, Subaru wishes to not love Seishirou, Wishes he had never met him.
Subaru's Wish is always the same:
Sakurazuka Seishirou.