An important issue which I haven't really discussed before is the structure of our desire set, and how we are to judge the relative importance of various desires when assessing well-being. Mere "felt intensity" will not do, because our strongest motivation may be for something that is harmful to us (e.g. the cravings of a drug addict).
Further, one might induce and fulfil extremely strong desires by deliberately putting someone in an undesired situation and then "saving" them from it. But surely you do me no favours by torturing me and then -- to fulfil my strongest desire -- stopping. But this example might plausibly be explained (away) as violating my strong desire to not be tortured in the first place. So let's instead use Parfit's example (p.497) of an addiction which induces a strong desire for the drug each morning, but has no other consequences (the drug itself provides no pleasure or pain). Should we force people to become addicted to this drug (and provide them with ample supplies)? Surely not! But summative desire theories cannot make sense of this. Sure, the person might have a moderate desire to not be addicted to anything. But that's not nearly so strong as the craving we will induce and allow them to satisfy every morning!
I think this relates to the counterfactual issues I discussed earlier. Holding the current situation fixed, what's best for me is to maximize the fulfilment of my desires. If I'm already addicted, then go ahead and give me the drug. But we might also step back and compare situations. Perhaps it would be best for me if I had never gotten addicted in the first place. Sometimes, rather than fulfilling an induced desire, we would be better off never having had that desire induced in the first place - no matter how strong it is!
But how are we to judge this? The natural response for subjectivists is simply to ask, "well, which situation would you prefer?" Since my global preference is to avoid the addiction in the first place, we should conclude that that is what would be best for me, no matter that the alternative involves greater desire-fulfilment.
So it is not enough to maximize desire-fulfilment. One must weigh desires appropriately, based not on their brute (felt or motivational) 'strength', but rather on their structural position within our larger desire-set. In particular, we ought to give priority to global desires, i.e. our preferences about (a part of) our life considered as a whole.
Success Theory excludes from considerations of well-being those desires which have no bearing upon our own lives. The guiding idea is that we are well-off to the extent that we successfully live the life we want to live. This naturally lends itself to privileging 'global' preferences over merely 'local' ones, since the former are explicitly concerned with the shape of our lives. As such, we can make sense of why maximizing local desire fulfilment (as in the case of induced addiction) might nevertheless not be what's best for someone.