Unfortunately, there is a fairly common idea that moral behavior comes 'only' from some higher authority. Many people DO believe that all the rules they abide by were dictated by God, Allah, Buddha, The Great Spirit ... whatever. Otherwise, they think, standards of morality would be merely arbitrary and would not be observed. Well.... what do they think IS the case?
Since 'believing' in a supreme being and rule giver is itself arbitrary, it is fairly easy (from the outside) to see that there IS a certain arbitrary nature to the decision(s) about what moral code, if any, to follow.

But some have never even thought about why basic moral principles are... well... basic... and practical. Everyone has been told as child.. "what if everyone did that?" (Like throwing trash out the car window, or drawing on the walls with crayons.)

Mother to misbehaving kid: "Johnny, please be good!" "Mom...I'll be good for a quarter!". "But, why can't you be good for nothing, like your father?"

But because the reasonable, practical side of morality seems so easy to get at, it is also an obvious topic for serious philosophical analysis. From the Greeks on, deep thinkers have worked on how to express the value of "being good for nothing"... or usually 'because it just works out better'.

One of the most detailed and complex works on the issues was Kant's Fundamental Principles of the Metaphysic_of_Morals It is a long dissertation saying that it is obvious to anyone who thinks hard enough that moral behavior is practical.

But perhaps the simplest form came from Mammy Yokum, the mother of L'il Abner, who stated clearly... "Good is better'n evil, 'cause it's nicer!"