Motivation refers to a process within a person or animal that causes that organism to move toward a goal or away from an unpleasant situation. Intrinsic motivation refers to the desire to do something for its own sake and the pleasure it brings. Extrinsic motivation refers to the desire to do something for external rewards, such as money and good grades.

Overweight and obesity are not simply a result of failed willpower, emotional disturbance, or overeating. Hunger, weight, and eating are regulated by a set of bodily mechanisms, such as basal metabolism rate and number of fat cells, that keep people close to their genetically influenced set point. Genes influence body shape, distribution of fat, number of fat cells, and amount of brown fat, and whether the body will convert excess calories into fat. The ob generegulates leptin, which enables the hypothalamus to regulate appetite and metabolism. The hormone ghrelin spurs appetite and leptin reduces it.

Genetics alone cannot explain why rates of overweight and obesity are rising all over the world among all social classes, ethnicities, and ages. The major environmental reasons are (1) the increased abundance of inexpensive fast food and processed food; (2) the increased consumption of high-calorie sugary sodas; (3) the rise of sedentary lifestyles; (4) increased portion sizes; and (5) the availability of highly varied foods.

Anorexia nervosa and bulimia nervosa are the best-known eating disorders, occurring mostly among young white women; however, a large percentage of all cases of eating disorders affect women and men of varying ages and ethnicities. Genetic and cultural factors influence eating disorders, but most are due to psychological causes, such as depression, anxiety, perfectionism, or a distorted body image.

All human beings have a need for attachment and love. Psychologists distinguish passionate (romantic) love from companionate love. Various brain chemicals and hormones, including vasopressin and oxytocin, are associated with bonding and trust; endorphins and dopamine create the rushes of pleasure and reward associated with romantic passion.

Two strong predictors of whom people will love are proximity and similarity. When in love, people form different kinds of attachments. Attachment theory views adult love relationships, like those of infants, as being secure, avoidant, or anxious.

Men and women are equally likely to feel love and need attachment, but they differ, on average, in how they express feelings of love and how they define intimacy. A couple's attitudes, values, and perception that the relationship is fair and balanced are better predictors of long-term love than are genes or hormones.

Human sexuality is not simply a matter of “doing what comes naturally” because what is “natural” for one person or culture may not be so natural for others. The Kinsey surveys of male and female sexuality and the laboratory research of Masters and Johnson showed that physiologically, both sexes are capable of sexual arousal and response. However, individuals vary enormously in sexual excitement, response, and inhibition. The hormone testosterone promotes sexual desire in both sexes, although hormones do not cause sexual behavior in a simple, direct way.

A balanced view of sexuality is that male sexuality is more biologically influenced than is women's, whereas female sexuality is more governed by circumstances, relationships, and cultural norms. Men and women have sex to satisfy many different psychological motives, including pleasure, intimacy, security, the partner's approval, peer approval, or to attain a specific goal. People's motives for consenting to unwanted sex vary, depending on their feelings of security and commitment in the relationship. Men who rape do so for diverse reasons, including narcissism and hostility toward women; a desire to dominate, humiliate, or punish the victim; and sometimes sadism.

Cultures transmit ideas about sexuality through gender roles and sexual scripts, which specify appropriate behavior during courtship and sex, depending on a person's gender, age, and sexual orientation. As in the case of love, gender differences (and similarities) in sexuality are strongly affected by cultural and economic factors. As gender roles have become more alike and women have become more economically independent, the sexual behavior of men and women has become more alike, with women wanting sex for pleasure rather than as a bargaining chip.

People achieve more when they have specific, focused goals; when they set high but achievable goals for themselves; and when they have approach goals (seeking a positive outcome) rather than avoidance goals (avoiding an unpleasant outcome). The motivation to achieve also depends on whether people set mastery (learning) goals, in which the focus is on learning the task well, or performance goals, in which the focus is on performing well for others. Self-confidence and grit both contribute to the attainment of goals. People's expectations can create self-fulfilling prophecies of success or failure. These expectations stem from one's level of self-efficacy.

Work motivation also depends on conditions of the job itself. When jobs are highly gender segregated, people often stereotype the abilities of the women and men working in those fields. Working conditions that promote motivation and satisfaction are those that provide workers with a sense of meaningfulness, control, variation in tasks, supportive relationships, feedback, and opportunities for advancement.

People are not good at predicting what will make them happy and what will make them miserable, and at estimating how long those feelings will last. Well-being increases when people enjoy the intrinsic satisfaction of an activity. Having intrinsically enjoyable experiences makes most people happier than having riches and possessions.

In an approach–approach conflict, a person is equally attracted to two goals. In an avoidance–avoidance conflict, a person is equally repelled by two goals. An approach–avoidance conflict is the most difficult to resolve because the person is both attracted to and repelled by the same goal. Abraham Maslow believed that human motives could be ranked from basic biological needs for survival to higher psychological needs for self-actualization, but this popular theory has not been supported empirically. Rather, psychological well-being depends on finding activities and choosing goals that are intrinsically satisfying and on developing the self-efficacy to achieve them.