I fail to see how Rikku has changed personalitywise.

It's logical that Yuna would follow her lead. Yuna is more or less a emotional sponge, and she draws a great deal from the people around her. (I'm the same way a lot of times -_-)

As long as she was in Besaid with the satisfied people there, X-2 would've never happened. The people there were happy, so she had herself thinking she was happy.

Kimarhi goes and finds the sphere and gives it to Rikku to show Yuna (because of course Rikku would go jet-setting across spira, she can't keep still for more than five seconds:P)

And then, hanging with Rikku, without the steadying effect of the other people who were her guardians, like in X, she starts absorbing the way Rikku reacts.

Remember when she's all 'poopie!' and they stare at her for saying something so unher? She practically says it's because of her being around Rikku.

And this fits in exactly with the way she was in X. If Tidus hadn't showed up, she would've never had another thought about going and dying for Spira. Rikku compounded the problem, which was probably why she got the courage enough to say No to Yunalesca.

It's just the way she is If it had been Paine she had hung out with for two years, she would be dressing in black listening to Emo^_^

Stuff happens, you're trying to believe that people can never change totally not right.