I haven't known Davey to give up on anything, and Brian doesn't think in terms of chances (ie 1st, 2nd, 3rd ...), he thinks more in terms of truth/honesty and personal integrity. While Davey has DUTY, Brian has LOYALTY and the memory of the proverbial elephant.
Ultimately, I'm jealous of Davey, Brian is a perfect fit to him, and Davey knows it -- he wouldn't be near as good a fit to me, I have too much weakness in my personality (as do most real human beings). The question of whether or not they are "destined" to be together is only an issue of how evil Dan is. There is no question that they are made for each other and belong together, the only question is if Dan lets them be together. Which, I guess, may depend on if Dan tries to punish me because someone got bit by a snake (like I believe Dan would change a storyline just to punish me for the actions of my "pet" ).
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Davey and Brian are 'made for each other and belong together' in a strictly literal sense. Dan created these people, and then did his best to show that they are a good fit for one another the first time around. If you believe that they are 'made for each other and belong together,' then it's a sign that you bought in to the Davey/Brian soulmates aspect of DO.
To the extent there is destiny, though, you are speaking of the all-powerful God that is creating these characters and then crafting what happesn to them...in other words, Dan. They are only 'destined' to be together if Dan believes that to be so. It is not really a question of evil or good. Evil would be creating people who are 'meant to be together' and then keeping them apart so they suffer. But then how was it ever their destiny if it can't happen? If you think Dan is trying to be 'good' or 'evil' in how he crafts his story or 'manipulates' his characters, then you may have lost the suspension of disbelief necessary to make the story work on its own.
Personally, I don't believe that in THIS world any one person is destined to be with any specific other person. While there may be a few people who are good fits, there is no such thing as 'made for each other' or 'belong together.' Who we wind up with is subject to a few coincidences in events and some choices that we make.
I have not seen any evidence that Dan as Author has created a world where people are 'meant for each other' and it is 'their destiny' to be together. I'm glad, because it makes it more interesting to see that people's choice have consequences and that we all subject to some degree of randomness that is not in our control. That is true for Davey, that is true for Brian. It's not good nor evil. It just is.