Desirism relies on a distinction between desires "to do" a particular thing and desires "that do" a particular thing.
For example, a desire that objectively satisfies the desires of others is not necessarily a desire to objectively satisfy the desires of others. The former is content to reach for something other than the objective satisfaction of other desires and to bring about objective satisfaction as a side effect, while the latter aims directly at objectively satisfying other desires.
Failure to observe this distinction lies behind one of the most common objections to desirism - the objection that desirism fails to justify the claim that "I ought to care about objectively satisfying the desires of others."
The actual answer that desirism gives is, "Perhaps you shouldn't. Even if you should, this interest in objective desire satisfaction would only be one interest among many and not the most important interest. There is nothing special about wanting objective desire satisfaction."
It is easy to explain how people make this mistake. A great deal of moral philosophy has been devoted to finding the one thing we should all care about - where all other values are derived from that. Proposals include eudaemonia (Aristotle), pleasure (and freedom from pain) (Bentham), happiness (Mill), preference satisfaction (Singer), and the well-being of conscious creatures (Harris). It is a reasonable first guess to assume that desirism follows the same general formula and simply nominates 'objective desire satisfacton' to this list. In this case, it is reasonable to ask what evidence there is that this proposal for an ultimate end succeeds where the other proposals fail.
However, desirism does not follow that model. Desirism holds that a state of affairs in which P is true has value for the agent who desires that P. For example, a parent concerned with the health of his child is not looking to maximize objective desire satisfaction. He is looking to create a state of affairs in which his child is healthy. He is acting to realize a state of affairs in which 'my child is happy' is true. This happens also to be a state in which his desire that his child is healthy has been objectively satisfied. To illustrate the distinction between "desires to do X" and "desires that do X," let us look at the simple case of Alph - a simple creature with one desire - a desire to gather stones. Alph spends his days on his small planet putting stones in a pile. Note that Alph does not have a desire that the stones be in a pile. An effect of Alph doing what he wants to do is that the stones will end up in a big pile. However, this is not something that Alph wants. What Alph wants is to engage in the activity of gathering stones.
Ultimately, Alph does not want the stones in big pile. He wants them scattered. If the stones are piled up, he has nothing to gather. It is only when they are scatted that he can do what he enjoys. The way things stand at this point, in order for Alph to gather stones, every once in a while he has to endure the chore (done purely as a means, not for its own sake) of scattering stones so that he will have stones he can gather. Now, let us introduce a second creature, Betty. Betty has no desires. However, Alph is given two injections to give to Betty. One injection will give Betty a desire to gather stones - just like Alph. The other injection will give Betty a desire to scatter stones. Clearly, if Alph gives Betty a desire to scatter stones, then he would not have to do any more work. While Alph is busy gathering stones in one area, Betty can be off scattering stones in another. This will allow Alph to continue to gather stones as long as Betty can keep up - and give Betty an opportunity to scatter stones so long as Alph can keep up. Note that Alph did not give Betty a desire to objectively satisfy Alph's desires. In this case that was not even an option. He gave Betty a desire to scatter stones. Betty's desire is not a desire to objectively satisfy Alph's desire to gather stones. However, it is a desire that helps Alph to objectively satisfy his desire to gather stones.
It is also the case that Betty can learn about desirism and still conclude, "I see no reason in here why I should care about Alph's desires. I see no reason at all to do anything but scatter stones. My only interest in Alph is that, because of his stone gathering, I get to scatter more stones." She would be right. Desirism does not argue for desires TO objectively satisfy the desires of others. It argues for desires THAT objectively satisfy the desires of others. It states that Betty has no reason to be interested in Alph's desires except insofar as she has a reason to preserve in Alph a desire to gather stones. If that desire was in danger of extinction, she would have a reason to protect it.
Let us assume that we keep adding members to the community. We end up with a community of thousands of members - some of them large and others small, some that move quickly and others that are slow, some with a desire to gether stones and others with a desire to scatter stones. As a new member gets introduced to the community, we can easily calculate whether there are more and stronger reasons to give the new creature a desire to gather stones or a desire to scatter stones. We simply need to look at which team is falling behind. If the stone-scatters are working faster than the gatherers, then the scatters have reasons to give the new person a desire to gather stones. However, the gatherers have no reason to care one way or the other. Consequently, the conclusion is to give the new member a desire to gather stones - a desire that tends to help objectively satisfy the desires of others.
In our complex world, the desires that help objectively satisfy other desires - the desires that people generally have many and strong reasons to promote - include such things as an aversion to lying, an aversion to taking the property of others without their consent, an aversion to killing non-aggressors, a desire to help those in need, a preference for true beliefs over false beliefs, and the like.