Humans are social creatures. In order to eat and reproduce more effectively we've found that banding together is the way to go! Unfortunately, earlier (and current) efforts to band together resulted in some pretty messy situations! For instance, you will find many examples in the history and current events of groups of people:
1 Person in a group of people taking all the food for himself/herself! This is known as stealing to some. It does this thing to a group of people making them all suspicious of one another and has the effect of DISBANDING the group and makes them not trust one another and therefore they work together LESS effectively and their food supplies dwindle and then their children die. We can assume that at some point some people decided that they'd rather not live like this, so they punished a person who stole.
You will find that the morals we have help us to exist as a society, to make a happier, more better place for us to live, eat, and play in. They encourage trust and social cooperation. But they can't be absolute, otherwise societies that have developed could never change based on a new threat and would then die off clutching their absolute morals defiantly to their bosoms.