In Freudian psychology, the pleasure principle states that an individual, guided by the libido and/or the id, will seek pleasure and avoid pain/unhappiness at an instinctual level. These impulses are primitive; the individual will often seek the most immediate means of satisfying pleasure and/or of avoiding unpleasure, even if this is not a good strategy in the long run. For example, when a person is hungry, they develop the psychic idea that they need food. The person, completely guided by the id will find food and eat it. But if they happen to be in an area where food is scarce, that person might just eat until the food is gone, not thinking to ration it out over the subsequent days.
Cue the ego and superego. The ego will take the id's impulse for immediate pleasure and decide whether or not it is a good idea. If the impulse is socially unacceptable, the ego might go through an anticathexis, a blocking of the impulse or a redirection: rationing the apple and planting (rather than eating) the seeds. The ego introduces the "reality principle" to the pleasure principle, thus making a more informed decision based on reason and social laws. The pleasure principle is necessary to life: we eat when hungry, seek shelter when cold, and so on. The pleasure principle applies to all urges and desires. So, when the id identifies a potential sexual partner, the ego steps in and guides the individual by engaging in socially acceptable rituals of dating, conversation, and so on.
Freud expands on the pleasure principle in talking of the "death drive." He supposes that since death is the absence of pain, there is a drive or impulse towards death just as there is the impulse to avoid pain (and seek pleasure) in life. Thus, oddly, he saw a link between pleasure and death.
The pleasure principle is the seeking of pleasure and avoiding pain to satisfy biological and psychological needs and this pleasure principle is the driving force that guides the id. The counterpart of the reality principle, this describes the "capacity to defer gratification of a desire when circumstantial reality disallows its immediate gratification." During infancy and early childhood this id rules by obeying the pleasure principle and at first the cravings are those of hunger and thirst but as they grow older, the id seeks out sex.