Throughout recorded history, the vast majority of philosophers have argued that people have an obligation to develop a sense of morality, to educate themselves how to make ethical decisions. After all, civilization cannot exist without people putting trust in one another every day, which would be utterly impossible if citizens did not have a reasonably high assurance that others would not take advantage of them. Getting on a bus requires trust that the driver will do everything in his power to deliver you safely to your destination; no one would do this if they thought there was a good chance the driver would take their money and drop them off in a dark alley, or drive the bus off a cliff. Thus, society cannot exist without trust, so every person should strive to educate themselves on ethical theories, so they can make morally justified decisions.
The study of ethics dates back to ancient Greece; Plato is considered the father of ethics, yet he was the student of Socrates, so many credit him instead. Ethics has always been in flux. Even Plato's own student, Aristotle, disagreed with him on what it meant to be moral. Although the study continued sporadically since then, great strides have been made in the past few centuries. Beginning with the Renaissance, European thinkers revisited the concepts put forward by ancient philosophers and later came up with entirely new theories, based on new influences, such as the teachings of the Catholic Church, as well as scientific advancements.
Virtue ethics sprang from Plato's philosophy. This theory holds that a person's character and motivations are much more important than his duties and human-made laws.
Aristotle, Plato's student, took a different approach, arguing that moral development is linked to a person's ability to reason and be a good citizen. He also advocated the "golden mean," or everything in moderation.
Immanuel Kant's "categorical imperative," which he formulated in the 18th century, requires a person to act only in ways that he thinks would be advantageous if everyone else did likewise.
Ethics continues to evolve. One of the most influential thinkers of the last century was Ayn Rand. Her objectivist philosophy called for people to serve no one but themselves because any kind of servitude was slavery. Other philosophers, like Peter Singer, are still writing today and exploring new ethical dilemmas, such as humans' moral responsibility to animals.
No one single ethical theory can be considered 'best," but all share the same goal of helping people make good decisions (think the "Golden Rule"). Therefore, people who want to further their moral education should study and understand the various theories of ethics. When a person has an ethical dilemma, he can consider one (or preferably several) ethical theories he believes make the most sense to help him reach a moral conclusion.