Because there are many more scenarios where she can give an opinion than other companions, save maybe Solas and Varric.
And again, because of this, you have to try to get her to hate you to the point where she starts drinking. I make all my in-game choices as I would do if I were the Inquisitor (ally with mages, keep the Wardens) with the exception that I decided to play a character who believes in the Maker for once, and I have seen many more approval notices than disapproval.
Every story decision I made, Cassandra disapproved (conscripted the templars, truce in the orlesian ball, kept the Wardens, did the ritual in the temple of Mythal, let Calpernia go and didnt drunk from the well), still had max approval at the end of the game. I think its impossible to have the drinking scene, unleash you really try.
No love for her here... sorry. I can't stand her and I never use her. A shame that I have to choose between 2 good characters when I leave the fade. I'd leave her there without hesitation. I loved every character since DAO but she already was a big turn off in DA2.
I'm in the minority here, but there is nothing I can do about it. I just cannot like her. 
Dont worry, its not like everyone have good taste in characters.