Why did Morrigan stay? A fan theory…
Flemeth volunteered her to save Ferelden with you. Even though she agreed to go in the end, she was first surprised, and then upset. It makes sense: mother was kicking her out of her home, and the safe place she had always hid from the Chasind from. Although the location had changed several times over her lifetime, Morrigan’s world was still a small one; it was always her and Flemeth, and although sometimes there were the men that her mother hunted, Morrigan had to go deal with the Chasind if she wanted any social interaction. And she didn’t understand their ways and norms, the things that seemed to come naturally to them.
When Flemeth rescues you and Alistair, it is Morrigan you wake up to. She is a source of answers and seems, for the most part, uninterested in your life. If this had been a decade ago, however, young Morrigan—the same girl who stole the hand mirror—would have asked question upon question, would have sought to understand you. And this, no doubt, would have irritated Flemeth—the same Flemeth who destroyed the mirror. It’s not that Morrigan lacked interest in the outside world—rather, that she couldn’t ask about it. Flemeth had instilled in her contempt for the outside world. Being on the receiving end of questions gives you power; asking them, likewise, does not. So she pretended, especially to herself, that she had no interest.
Morrigan talks as if the child who stole the hand mirror, died the day it was shattered. We know that that’s not true. As Morrigan matured, she learned to be comfortable with who and where she was. She may not have been in the same kind of battles that Alistair and perhaps even you have been in, but she was still a fighter. She has a very strong sense of the moral and immoral, of right and wrong, shaped around her own experiences and how she was treated by the Chasind. She claims disinterest in the world—and yet, she stays. She is touched if you gift her a golden mirror, like the one she had as a child. She also responds well to jewelry as a whole. It reminds her of who she could have been, if the mirror hadn’t been destroyed. It is the rebirth, in a way, of the young Morrigan who is interested in the world, though we know that Morrigan has just been in hiding.
Though it may have been the embittered Morrigan who joined the Wardens, it is a curious one who stays. Traveling as a warrior throughout Ferelden with the Wardens, gives her the motivation of saving the world, while also fulfilling her desire for understanding. She originally agreed out of obligation, but don’t let her fool you: she stays because she wants to be a part of the world. Apostate that she is, that’s not something she could get anywhere else.
Not that she would tell you this, but that’s why Morrigan stays.