Dzongsar Jamyang Khyentse Rinpoche, author of What Makes You Not a Buddhist provides four criteria to consider. To be a Buddhist, one must believe in all four of these tenets or seals. These are: 1) all compounded things are impermanent, 2) all emotions are pain, 3) all things have no inherent existence, and 4) nirvana is beyond all concepts.
2) All emotions are pain. Certainly pleasure would not be included. But Rinpoche says "all." But how could pleasure be pain? Every experience contains its own end, especially pleasurable experiences. We may want to hold onto pleasure; to not let it go. Trying to hold on contains the seeds of dissatisfaction or dukkha. Dukkha (often translated as "suffering") pervades everything that we have a vested interest in. Whenever we are awake to what is happening this arising and passing away becomes apparent, painfully apparent. Once again this is impermanence, the first of Rinpoche's criteria. I am inclined to agree that desire, if I can rephrase slightly, is tinged with pain. This principle leads to a misconception about the Buddhist approach. Is Rinpoche advocating an emotionless and desireless way of being? I don't think this is exactly accurate. It would be more accurate to say that we can become more flexible with our emotions and desires. It would be less painful if we could have fewer expectations. Another misconception claims that having fewer expectations results in having fewer goals. Not necessarily so. We still aim towards goals; we still act in the world. When things don't go as expected we respond to reality proactively. What becomes absent is the self-referencing drama about how we didn't get what we wanted, how the world is cruel and capricious, and how the future is irrevocably tarnished.
Another way to interpret this seal is to look at the meaning of "emotion." In a chapter written with my late mentor, Terrance Brown, M.D., a distinction is made between emotions and feelings. Emotions are highly charged and disruptive experiences, both positive and negative. They arise when our capacities to cope are overwhelmed. The emotions restore equilibrium albeit in a primitive and expensive way. As we develop through infancy into adulthood, we become less emotional; emotions arise less often because we have more sophisticated ways of coping with situations (namely intelligence). Feelings in contrast to emotions are ubiquitous and serve the function of helping us to navigate in an uncertain world. Confusion arises because all emotions contain intense feelings but not all feelings rise to level of emotions. By making this distinction between emotions and feelings, we can now revisit the seal "all emotions are pain." This certainly would be the case. Emotions by their very nature are painful because they arise when we can't cope with reality in that moment. They are primitive, disruptive, and expensive. The goal of emotional development is to get rid of emotions altogether and to refine our capacity to use feelings and intelligence to problem solve.
(Brown, Terrance A., and Arnold Kozak. 1998. Emotion and the possibility of psychologists entering into heaven. In What develops in emotional development?, ed. Michael F. Mascolo and Sharon Griffin, 135-155. New York: Plenum Press. )