Self-centeredness has always been Yuki's overriding character trait. The very first time we saw her she hasn't seen Zii in quite some time, but she's like, "Did you dress as Tralala for me?" And even when she was crushing on Gary and we were supposed to feel sympathy for her, it was still all about what she wanted. What did Gary want? Who cares. She never bothered to consider it because it was irrelevant in Yuki world.
To be fair, though, most of the characters are completely self-centered. The only one of the girls (or guys) chasing Gary who ever stopped to ask what he wanted was Kiley, which is why she's my favorite.