Emotions are consequent upon desire and belief not freestanding mental episodes. The subject has certain desires, such as the desire to stay safe, and also certain beliefs, such as the belief that a looming animal is dangerous, and the result is fear directed at the animal in question. It is hard to see how an emotion like fear would be possible without antecedent desires and beliefs of these kinds. Similarly, an individual has the desire to form loving relationships with other individuals and believes that this one is a good candidate; as a consequence love blooms in that individual’s bosom. You want something and you believe this thing will give it to you; a suitable emotion supervenes. The desire and belief are necessary conditions of the emotion. What sense would it make to have emotions in the absence of desires and corresponding beliefs? Don’t emotions exist to serve desires—wishes, needs, appetites? Presumably they evolved with that purpose: they are the servants of desire, with belief as the method of tying them down.  They are desire-dependent. But desires are not emotion-dependent: an organism can have needs and desires and no consequent emotions. Do all animals have emotions? Unlikely: consider insects, snakes, sharks, lobsters, etc. Here it seems plausible to attribute needs and desires, as well as sensations and perceptions, but no emotions—no love, hate, fear, joy, sadness, etc. Or consider that Vulcan animal Mr. Spock, chief science officer of the USS Enterprise: he is equipped with standard terrestrial psychology except for emotion. He has his needs and desires, his hopes and wishes, his values and aesthetic sense; but he feels no emotion, not a jot (much to Bones’ consternation). Mr. Spock is logically possible and perfectly intelligible; there may well be rare humans in like case (just destroy the amygdala). So we have an asymmetry between desire and belief, on the one hand, and emotion, on the other, with emotion emerging as the derivative phenomenon.
This reflection might lead us to suppose that emotion is reducible to desire and belief: an emotion just is an appropriate desire-belief pair (compare reducing intention to belief and desire in standard belief-desire psychology). If a creature desires to stay safe and believes that avoiding a certain animal is the way to do it, then this creature will feel fear towards that animal. But this is very implausible because the emotion is not functionally and phenomenologically equivalent to the desire-belief pair: the emotion is a feeling that triggers certain distinctive sorts of behavior. There is indeed a lawlike connection between the two, and a desire-belief pair is necessary for emotion, but it is not logically sufficient for emotion—not what emotion consists in. Emotion is a genuinely distinct type of mental state over and above desire and belief (the same thing is true of intention). It depends on desire but isn’t a type of desire. Spock lacks this type of mental state though he is not bereft of the other types; he has a localized psychological lacuna. Captain Kirk throbs with something Spock genuinely lacks; he has something Spock doesn’t have just in virtue of having desire and belief. Still the connection is intimate, which is why Spock is otherwise so similar to Kirk: he has normal desire and belief, he acts much like a regular human, but he lacks this one psychological trait. He is preternaturally calm, is not given to humor, and speaks rather slowly, but he isn’t completely alien to us—he is our conative twin (well almost). He is living proof that there is more to emotion than desires modulated by beliefs; but he also tells us that emotion is not the be-all and end-all—it can be removed without drastic psychological impairment. 
This presents a puzzle: why does emotion exist? Spock does perfectly well without it, even better in some respects than the average emotional human. For one thing, he is always rational. Why would the genes engineer a psychological trait that causes irrationality? What good is that to survival? Yet emotion is common, especially among mammals; so it must have some useful function. Emotions often make us feel bad, take the wind out of our sails, and interfere in our life-projects—wouldn’t we be better off without them (that is certainly Spock’s opinion)?  It is true that they can imbue in us a sense of urgency, especially the aversive emotions, but couldn’t that be supplied by desire all by itself? The downside can easily seem to outweigh the upside, and the functional role of emotion seems not logically unique to it (Spock is perfectly capable of prompt decisive action). Sharks are pretty successful survival-wise and they are emotionless killing machines (unless they harbor a soft fuzzy side that we never witness). A pure desire-belief psychology seems both possible and advantageous, sans emotion. Nor does it look as if emotion is some kind of evolutionary remnant or contingent side effect: it presents itself as vital and vigorous. But the principle of its adaptive value is elusive and perplexing: emotion is a biological puzzle (like sex, consciousness, creativity, altruism, etc.) You can build an excellent survival machine employing only desire and belief (or more primitive analogues of these), so why insist on installing emotion too? We humans find our emotions tough to deal with—and it must be admitted that we are more than usually replete with them—so why equip us with so much emotional baggage?  Why aren’t we more like Mr. Spock? Why aren’t we affective zombies? Let’s call this the Spock Problem—the problem of why emotions exist in so many animal species. They seem surplus to requirements, inherently prone to pathology, and dubiously functional; yet they are extremely widespread in the animal kingdom. What is going on? Why emotion?
 I focus here on the connection between desire and emotion, but we should not forget that emotion is also highly belief-sensitive. Human emotions, in particular, are shaped by thoughts and theories, opinions and ideologies. Other animals do not have emotions that are so cognitively laden.
 Emotion is no doubt largely innately based not acquired by learning and instruction. There are thus human emotional universals. It isn’t that Spock was brought up in a culture without emotion while Kirk was: both have their emotional make-up (or lack thereof) as a matter of genetic endowment. Why did Spock’s evolutionary background lead to an absence of emotions while Kirk’s left him with a plethora of them?
 Emotions seem most common in social species, suggesting that their function has to do with living successfully in a social environment. Might they result from sexual selection not natural selection? Do they operate mainly as signals to others not as motivational factors in their own right? We humans have a rich emotional life (too rich!) and we are also a deeply social species; most of our emotions are social (love, hate, envy, jealousy, pride, shame, disgust, etc.). Emotion is the currency of social life. We often evaluate each other by reference to them.