This little sob story doesn't make me feel sorry for her at all
100%. Like
d0shie said, it's a regularly practiced attempt at playing up the interest of the third wheel and making her a more credible potential love interest. It doesn't--like most other attempts--change anything, however; this is especially because of something Shiori reflected on:
They were changing.
Yes, that's right: see, at an elementary school age, what happened before Shiori and Jirou parted ways would make sense (in fact, Jirou's action would be somewhat exceptionally
bold); but they'd gotten older by the time of that parting--and were just about to enter
high school. That excuse has long faded, because they'd both aged (a
lot can happen to spur growth in just those few years, as Shiori recalled--and, in keeping with that, it doesn't have to be something good) and matured to some degree.
Or, at least, Jirou did. Jirou, despite his overwhelming feelings, did what a male is expected to in a situation like this--he approached the girl; Shiori, despite
actually having always been romantically fond of Jirou (I had a feeling this was the case),
chose to shut his advance down and friendzone him. If you reversed the positions in that exchange, and just simply had Jirou dillydallying about instead of making the move at that critical juncture; and, as a result, Jirou lost his chance and Shiori ended up with a good man, then mark my words:
no one would question it. Shiori had her chance--she
had Jirou, and then decided with her actions she didn't really want him all that badly after all.
That is, until Akari came along.