I'm not entirely sure. I sided with the Templars but had very few points on the friendship/rivalry scale (kept flipping on the be friendly/annoy her routine). I was a Mage for that run through so I don't know if that made a difference.
For a warrior (Templar) runthrough she joined up with my Hawke.
EDIT - One bug I had (Repeatedly) on the Mage run was that every time I went into the Hanged Man, it reran the Danarius choice so I ended up getting rid of someone about ten times