As we all know the joining can be fatal for some. The concoction a recruit drinks is a special blend of darkspawn blood, lyrium AND Archdemon blood, so it is no wonder people die, but what attributes affect the survivability rate? Why do some people die while others survive?
As far as I know, there is no explanation for why certain people survive and others don't.
The implication is that people are affected by the poisonous nature of the darkspawn blood different, much like humans in the real world are affected by poisons, venoms, and even many pathogens differently. Those people who are not "strong" enough (mentally, physically, whatever) succumb to the joining and die.
But exactly what that "strength" is, is never mentioned. It's even possible the Wardens don't actually know the reason. Certainly none of the Wardens we see during the ceremony say anything like "if you are pure of heart, then you will survive", or anything like that.
In fact, I'd assume that if they did know what it was that let some survive and others die, they'd probably work harder to weed out the unsuitable candidates long before they got to the Joining.
The common connection seems to be that those who are selected are generally more fit and hardy, and by and large seem to also possess a bit of the whole "the ends justify the means" attitude, though Alistair would be one of the exceptions as far as I can see. However, we don't have a definitive answer.
For example, I believe World of Thedas Volume 1 mentions that while there have been rumors of more Qunari/Tal-Vashoth attempting the joining, there is no confirmation if they survive or not. While this could be attributed to the rumors and headcanons about their blood, it is hard to deny that most of their species are hardier and more fit than most in Thedas. And with their mindset, they would certainly make great recruits, but we don't know what happens with them.
Overall, it's not very clear. But it will probably not be answered, as it leaves open a lot of room for interpretation and would take away a bit of the mystery of the Wardens.