Perception, thought , imagery, and memory, are important causes of emotions. Once emotions is activated, emotion and cognition influence each other. How people feel affects what they perceive, think, and do, and vice versa.
There is whitespread agreement among scientists that emotions are important in individual development, physical and mental health, and human relations.

"The movements of expression in the face and body, whatever their origin may have been, are in themselves of much importants for our welfare. They serve as the first means of communication between the mother and infant; she smiles approval and thus encourages her child on the right path, or frowns disapproval. We readily perceive simpathy in others by their expression; our sufferings are thus mitigated and our pleasures increased; and mutual good feeling is thus strengthened. The movements of expression give vividness and energy to our spoken words. They reveal the thoughts and intentions of others more truly than do words, which may be falsified."

The Expression of the Emotions in Man and Animals (Charles Darwin)

Hate, love, and jealousy are among the most important human emotions.

Hate is a common emotion, with many causes, both rational and irrational, and its results can range from negligent to enormous. Minor hate can be caused by many things, such as disrespect, rudeness, unfairness, or abuse. This kind of hate is generally not dangerous, however it causes stress and discomfort, and inhibits the development of relationships. It can also be dangerous to a person's psychological health.
There is also a more dangerous kind of hate, a deeper hate which can lead to the development of cults, terrorism, and genocide. Dr. Robert Jay Lifton, psychiatrist and professor at The City University of New York, describes a process that leads to this kind of hate. It involves extreme historical trauma, confusion and chaos, followed by a group with a revitalizing ideology that becomes genocidal by feeding on an impulse to destroy the designated victim (Lifton N. Pag.). This is the kind of hate that has lead to such disasters as the Nazi Holocaust, and is the cause of hate crimes which happen every day all over the world.

Love is an important emotion because it facilitates the development of human relationships. The human sex drive is an important cause of attraction, which often leads to love. However, where love comes from, and why it arises, are questions that are not well understood, and are often the subject of poetry and song.

Jealousy is essentially a simple emotion. It arises when one person has something which another person desires, such as wealth, property, intelligence, or personal relationships. If the desire is strong and is not suppressed by respect or fear, it leads to jealousy. Usually, jealousy is not harmful, but if it is strong it can evolve into hate and can lead to theft, assault, and other crimes.