Why Ryuuken decided to go into the bookstore was a mystery to even himself. If he really wanted to read a book, why bother going into a brick and mortar bookstore? Ryuuken's whole state of mind required him to believe that he held no love for antiquated systems. It bothered him even more that he felt such a rush of irrational emotions all due to a simple book display. But then Ryuuken had never liked Shakespeare's Romeo and Juliet, for it reminded him too much of the story of his friends.
What gave Juliet life is that for all her beauty, she is a stubborn naïve child. Masaki was beautiful, smart, talented and charismatic. But only a child, a besotted son, would fail to see her childish youth and naïvety. Masaki knew that this life of domestic bliss that she had created with Isshin was only a temporary illusion, but she did it all anyway. Even when she knew that this would hurt them: Isshin, the children, him. It is hard for Ryuuken to tell whether it was Juliet or Masaki that was more suicidal in their love.
And Romeo, the man who stole Juliet's heart, was he not just waxing poetry about another girl a few acts ago? Of course the equally dramatic Isshin would probably love the comparison in spite of that implication. Even now the showy way that he clings to this romantic image of Masaki and their relationship, well it made Ryuuken sick. Isshin was supposed to be the enemy or at least the adult. Why did have to be so hopelessly, and even at times selflessly, in love with the heroine?
Unlike their counterparts, Masaki and Isshin didn't die in their climate fight to break away from their pasts, and so they got to live the sequel Romeo and Juliet so desperately wanted, "Life in Exile". But life in exile is not meant to be a happy ending. Life in exile means constantly watching over your shoulder, just trying to enjoy every moment you have because the chase never really ends. The fact that their lives were cut so early gives the illusion that if only Romeo and Juliet would have survived that one night everything would have turned out fine. Masaki's death is a perfect illustration to the error in that logic.
Perhaps the most unsettling part is that Ryuuken never quite knew where he fit in the story either.
In the beginning, he might have cast himself as Paris, the one that Masaki should have loved. Everyone knows what happens to Paris, and Paris does not end up helping the unlikely couple. Paris does not become Romeo's best friend even years after Juliet dies.
When Ryuuken looked at Romeo's friends, he felt a certain something lacking. During his angrier times he could see himself delivering Mercutio's dying soliloquy. He could understand a dramatic "curse on both your houses", but anything Queen Mab would be crossing over into Isshin's territory. Ryuuken could be dramatic but he would let Isshin play the jester, the free-spirited fool. Perhaps the more calm and practical Benvolio would be a better fit, but Benvolio is Romeo's cousin. Just admitting that Isshin was his friend was like pulling teeth. He would die of shame if he had to say he shared some genetic material with Isshin even if it was only in idle comparison.
In the end, Ryuuken best identified with the nurse. The nurse told Juliet she was being stupid when she was, but she helped when Juliet needed help. The nurse is tied with the Capulets, but not really part of them, just as Ryuuken, who was raised by his crazy father, was tied but never really part of the larger Quincy community. The irrational, but most personally compelling reason is the irony of the choice. The nurse is after all a comedic character known for saying the most ridiculous lines, and Ryuuken can't help but think that in the larger scale of things some God is writing his story and laughing because he despite all his stoicism he still manages to be this world's perfect comic relief.
He buys the book, internally writing off the whole incident as a brief moment of sentimental insanity. If nothing else he can give the book to Uryuu for some holiday.
Author's Note:
I don't own anything. Otherwise the Isshin &Ryuuken omakes would be canon. Can't you see Ryuuken just coming up to Uryuu and being like "Happy National Lima Bean day. Here is a book" and then leaving. This of course would greatly confuse Uryuu and after the shock wore off would lead to a wonderful mess of funny faces…Yeah you don't even know all the stuff I think up.