Selfishness is the default and the natural impulse. You rarely have to convince people to take the selfish option, unless they are long indoctrinated to be selfless.

The objective reason to act morally is to live in a society where you have a chance to receive civility in return. Common morality, for example “don’t murder”, is about avoiding a continuous escalation of bad actions that are likely to come back to you.

It’s about having a functioning society, rather than a prison where you are constantly looking over your shoulder to avoid being shived by the brother of the guy you murdered.