To answer this question we have to keep in mind people have different meanings of “what is the good or what goodness is?” ‘Why should I be good?’ is an ethical question. It implies that an appraisal has to be been done by myself and or others of my behaviour. This appraisal would determine the right things I have done, thought, or felt. From this, we can therefore formulate the moral law. Therefore there must be a lawgiver. To refer to “the right things,” I am suggesting there are ‘wrong things’ too. For how can I affirm one, unless I believe that the other exists? The next challenge is what is right for me, may not be right for another person. This is the attitude the world is pushing and even doing so to our children in high schools and youths attending colleges or universities.
Ethics, according to The Cambridge Dictionary of Philosophy, is “the general study of right action” which “concerns the principles of right and wrong that govern our choices and pursuits.” (p. 286). The German philosopher, Immanuel Kant, as explained in A Brief History of Western Philosophy, says, “the only thing that is good without qualification is good will” (Kenny, A. p. 270). He says to act with duty is to exhibit this good will. Still he has not said what good means and by what standards are we to judge it.
Duty implies purpose. This purpose is beneficial to both the individual and others. Purpose seeks peace and activates goodness. It motivates us to fulfill our natural duties in agreement with the moral law. Now which moral law is Kant talking about? For if morality is different for different people then Kant maybe referring to some moral law that might be universal or accepted by all people. But here we have two ideas: (1) a moral law that is different for different people and (2) a moral law that is accepted by all persons. Now we have to wonder how did this moral law came about, for all people. Also we still have to be clear on how moral laws for different groups of people or different cultures or religions come about.
Clearly we have a problem here. We have a problem of different standards. Can all these standards be right? Should there be one universal standard of right and wrong? Who will decide on these standards? God? What about people who do not believe in God? Whose standards will they accept or live by? Can we have goodness or can a person be good without God or belief in God? Who is to judge or decide on this question? Clearly there are many activities that are right for a Muslim, Hindu, Christian, Buddhist, Zoroastrian or Jew that vary. There are some that are common like “don’t steal;” “don’t kill” and so on. So how can we deal with the problem in society? How can we truthfully be fair to all people—no matter their country, class or creed? This question every individual has to deal with for it is real. So the question is not only, “Why Should I Be Good?” It is also, “How Can I Be Good?”
For me as a Christian it is simple. Be Christlike. The Bible says, "Imitate God," (Ephesians 5:1). The apostle Pauls says, "Imitate me as I imitate Christ," (1 Cor. 11:1). The Bible is God's Word, (2 Tim. 3:16). God's Word cannot be broken (John 10:35). But others will disagree of course.