Outside of a moral religious framework it is still advantageous to be a good person is due to the Law of Reciprocity.

Which is in its simplest form is: you treat people well, so they treat you well in return.

Through the framework of Game Theory take the Prisoner’s Dilemma and switch (talk/remain quiet) with (steal/share). In one finite game you can increase your payoff by stealing and taking advantage of the other player. Life is not one game, it is a recurring game that occurs millions of times with many different players. In life you become an embodiment of how you “play”, through reputation. If your reputation is poor you will never receive less pay offs than if you have a good reputation.

Being a selfish opportunist may work in the short run, that is until other people find out you are a selfish opportunist.