>>233697468Sayaka's character is much, much more than simply "durr she do dumb things for boy she likes", and she isn't just some selfish bitch either.
When she meets Mami, she idealizes her as exactly what magical girls should be like. A selfless protector of the people. What Sayaka does not realize at the time is that Mami is not at all what she chalked her up to be, and Mami dies too quickly for her to ever realize that Mami was manipulating her and Madoka into becoming Magical Girls so that she wasn't alone. So she has this lasting image of a magical girl who gave her life to protect everyone in her mind... which was flawed from the start.
Sayaka makes up her mind after a long while considering, this false image of Mami's death combined with Kyousuke's outburst being what finally pushes her over the edge. She wants to use her wish to help someone else, and does so. She's incredibly happy about it all.
And then, reality hits. Another magical girl, Kyouko, moves into town. She immediately comes off as selfish as can be, which pushes Sayaka's buttons. She's infuriated that someone like this proclaims themselves to be in the same position as Mami while letting Familiars run away so that they'll mature and drop a Grief Seed later. She's told the merits of being selfish, yet still can't bring herself to accept that stance. Now, her perception is even more skewed -- she sees Grief Seeds as a reward that she shouldn't be taking, because that would be selfish. Since the cloudiness of one's Soul Gem is directly connected to their mental stability, not that she knew as such, this sets her down a dark path.
On top of all of this, the further secret of how magical girls operate is revealed to her in such a rough time. All of her illusions about the greatness that was Mami are crumbling apart, and she's left with having traded her mortality for someone else while everything and everyone is telling her that it was a huge mistake.
