What was the real reason that Anna wanted to make her own choices?

On the face of it, it seems that Anna's reluctance to continue acting as a donor for Kate is due to her wanting to pursue a life of her own. Anna's still young and full of life; she wants to enjoy herself, play sport, and hang out with her friends. But things are never quite so simple in My Sister's Keeper, as we discover during the legal hearing arising out of Anna's decision not to donate her kidney to her dying sister.
For it is here that we find out that the relationship between Anna and Kate isn't quite what we thought it was. It's not that Anna's reluctant to donate to Kate; it's that Kate doesn't want to be the recipient of her selfless giving. In not donating her kidney to Kate, Anna is fulfilling her sister's wishes. The truth is that Kate feels like she's become a burden to Anna; she hates the fact that Anna's always being called upon to act as her savior. Just for once, Kate would like to be the savior, instead of feeling weak and helpless all the time. And by getting Anna not to be her donor, Kate can be that savior and can feel that she's doing something for someone else, rather than always being the supplicant.