Numerous reasons:
#1. Such acts are deleterious to society: they disrupt the security and safety of others, they lower production as a result, they cause material damages to companies, leading to lesser tax-income to the government and cause expences to society. Acts leading to increased workload and amount of suffering for a larger group of people than the people who stand to gain from the act, should be discouraged.
#2. Quite understadably there are punisments for doing 'bad' things. Such punishments and the reputation accompanying the punishments serve to do me more harm than any coiceivable 'bad' act could bring me good. In essence, this is a result of a simple cost-benefit analysis.
#3. I would not wish such acts to be visited upon myself. I wish to live a secure, stable and free life like most others. If others were to act in 'bad' manner and do things resulting in deleterious effects upon me, I would feel sad and angry. I know how uncomfortable those feelings are and because I emphatize with people, I would not voluntarily visit such misfortunes upon them.
#4. I believe it inherently wrong to hurt others or bring them misfortune, for simple personal gain.
#5. To me, life is precious and wondrous. We get only one shot at it: once we die, that's it. Due to it's transitory nature, life is all the more valuable and should be nurtured and protected. I believe that once we die off, we should leave mankind and society slightly better off than what it was before we were born. This way, our decendants will have a better change to live a good, secure and free life, able fulfill their goals to a greater degree than we did. That in itself is a worthy goal, I believe. To act in a 'bad' manner, to break the well-founded laws of society and to give jack s**t about others is not conductive to reaching this goal.
Therefore, I act in what I perceive to be moral manner.
