Why do some people exercise faith and others not?

Under synergism, man cooperates with God through prevenient grace. Thus, salvation is still of grace, but the grace is resistible. But what is the explanation for one person (Regan) resisting this grace, and another person (Anderson) not?

Either the reason must lie in the nature of the grace given, in the nature of the person it is given to, or in neither. But none of these are satisfactory answers:

A. The reason cannot be because of the nature of the grace given

1. Firstly, for God to give more or better grace to Anderson than to Regan would make God a respecter of persons. That seems unacceptable in a theology where God loves all people equally and truly desires them all to be saved. (Also Acts 10:34, Dt 10:17 etc.)
2. Secondly, synergists typically hold that God does everything possible to save everyone. But if he gives more or better grace to Anderson, then plainly he didn’t do everything he could to save Regan.
3. Thirdly, for the answer to lie in the grace implies that the grace is irresistible for those who are saved. Otherwise, the grace itself is not the reason they respond affirmatively—its nature does not really explain their response.

B. The reason cannot be because of the nature of the person

1. Firstly, synergists are quick to deny that Anderson is saved because he is better in some way than Regan—for they are anxious to show the monergist that their position doesn’t contradict Ephesians 2:8–9. If the explanation for our faith is that we are smarter or more discerning or more morally virtuous than unbelievers, or something like that, then it seems to really be of our own doing—in which case we have cause for boasting.
2. Secondly, prevenient grace is supposed to overcome these sorts of intellectual and moral failings, precisely to enable all people to respond affirmatively to the gospel. So if Regan failed to respond because of an intellectual or moral failing, while Anderson did not, then insufficient grace was given to Regan—leading either to the same problem outlined in (A); or to the notion that prevenient grace cannot overcome the noetic effects of sin in some people.

C. The reason cannot be neither of the above

1. To say the reason does not lie in the quality or quantity of grace given, or in some virtue or otherwise of the person to whom it is given, is simply to say there is no reason at all. But surely that won’t do. The Principle of Sufficient Reason has overbearing intuitive support for contingent states of affairs such as these.
2. If we try to salvage this by appeal to some external reason that does not lie in the grace or in the person, then surely that reason is not relevant to salvation at all. Yet it seems grossly unfair that the reason Anderson is saved while Regan is not has nothing to do with any of the pertinent factors to salvation, but rather to some outside factor beyond their control. Indeed, that’s obviously unacceptable to someone with a libertarian view of freedom, since it removes responsibility for one’s response to the gospel.
3. Lastly, if we try to defuse this problem by saying the reason is simply that Anderson chose well while Regan did not, we go in a circle. Why did Anderson choose well? What is the explanation for that? Either there is a reason that relates to (A) or (B) above, or it’s random. But it can’t be (A) or (B), and it obviously can’t be random, since it’s hard to see how it could be Anderson’s choice if it were—not to mention this would turn salvation into a crapshoot; a matter of blind luck.

You might think I’ve missed a possibility: perhaps the explanation is a combination of (A) and (B). But how could combining two unacceptable answers give us an acceptable one, rather than a doubly unacceptable one?

So what is the reason some people respond to prevenient grace while others reject it?