In any Bioware game, kindness is a currency. You make regular payments to earn affection points. You pay by listening to someone talk about their problems and giving them cheap gifts. Put in enough coins and sex falls out.
After that, the relationship is over. Sex was your end goal, the reason for everything you did. There was no reason to stick around after you got it. That was what you were paying for.
It doesn’t matter if you find Morrigan’s Ayn Rand inspired morality to be utterly repulsive. Just sit there, nod your head, and she’ll fall for you. It doesn’t matter that your Shepard is a paragon of virtue and integrity. If you play at being anything but when Jack’s paying attention, she’ll love you someday. In these games, telling women what they want to hear and listening to their problems is all it takes to get laid.