Many and various forces, designed to keep people more dependent on others than is strictly necessary, are constantly at work:
Cultural Demand. Many different cultures traditionally define sex roles in a hierarchical fashion, with male roles and responsibilities seen as superior to female roles and responsibilities. In traditional American culture (as diverse as that beast is), it is typical for the male to be the sole economic support for the family and to take the dominant leadership role in running family affairs. The female role is more concerned with making a home and raising children. In this traditional scheme, wives are dependent on their husbands for money, and generally defer to their husbands when it comes to making decisions. These traditional roles did not come out of a vacuum, but rather were justified and promoted on religious and biblical grounds. God created Eve (the female) as a helpmate for his first and primary creation, Adam (the male); that sort of thing. The presence of these role demands places social pressure (sometimes very heavy social pressure) on women to conform and live out their lives as dependents of their husbands.
Parental Demand. People also sometimes pressure other people to become dependent on them by acting in ways that create strong, often emotional demands upon those other people to conform. Parents are often accused of "guilting" their adult children into seeing them more often than the children might otherwise want, for instance. Such children often comply with their parents demands, not out of love so much as out of a sense of duty or responsibility that may, in fact, be misplaced.
Parents may also raise their children very strictly and teach them that they are not capable of functioning without parental approval. Children from such homes may rebel and cast off parental authority, often in self-destructive ways, or they may grow up to be conforming adults who continue to look for approval from their parents and other authority figures.
Partner Demand. Relationship partners (spouses, lovers, friends, etc.) also sometimes act in manipulative ways so as to compel their partners to behave in ways they desire. They may withdraw themselves emotionally from the relationship, for example, or act abusively until they get their way. Because affection (belonging) and safety are a fairly basic needs that people are always looking to meet to one degree or another some number of people cave in to such demands, seeing this capitulation as better than their other alternatives.
Codependency. Some relationship partners get into what are called "codependent" relationships with their partners, who are often addicts, or otherwise selfish-acting sorts of people. Codependent people put their partner's needs before their own, and in so doing, enable their partners to engage in further selfish actions. A variety of motivations are at work in codependency. However, a central belief shared by many "codependents" is that their own worth is measured by how well they can care for (or control) another. By building a responsibility to maintain the health and welfare of another into their own self-concepts (something they can never fully control or determine), codependent people insure that they can never be in control of themselves. Codependency thus frequently arises out of a misplaced sense of duty.
Internal Factors. Sometimes there is no particular outside demand for dependency, but people are drawn towards it anyway. Some people are intimidated by the demands of adulthood (the need to support one's self, etc.) and look to others to provide for them. Other people have developed a sense of self-worth that is defined entirely by what other people think of them. Such people end up being dependent on their partners (temporary and permanent), and will do most anything to get those people to approve of them.
Dependency is not a bad thing in of itself, but it can become a bad thing when adults are too dependent on others to meet their needs.
Self-Control. When you are dependent on another person, you are at the mercy of their moods and whims. Your own peace of mind becomes dependent on theirs. A certain amount of independence and detachment from others is thus necessary for you to ever have a reasonable expectation of peace of mind.
Mood Control. Dependency beliefs are related to your emotional health. The more dependent and helpless you tend to believe you are, the more likely you are to become depressed, or alternatively, frustrated and angry. The more you can find ways to express your independent autonomy, the more your self-confidence tends to grow and the more relaxed, happy and fulfilled you tend to feel.
Financial Control. When you are in a dependent position within a marriage, you can end up taking on debts and obligations that you don't approve of. Your spouse may buy a car that you can't afford. Payments on the car may go onto credit cards that you build up debt on. Your acceptance of the situation makes you unable to save for retirement, or to save enough money up to purchase a house (instead of renting). Your lack of investment foresight and financial discipline come back to bite you when you are older and ready to retire.
Fear Control. Becoming a less dependent person tends to make you also a less fearful person. Dependent people believe (often mistakenly) that they cannot survive without particular other people in their lives. Alternatively, they may believe that they can survive without those particular people, but that life will not be worth living. Usually, these are mistaken beliefs. While it is true that people generally do need other people to be happy, it is not true that any given particular person is necessary. New relationships can be formed that will satisfy just as well or better.