One of the greatest enterprising INITIATIVES shown by David was that he did not graze his sheep or feed his goats with the companionship and the comfort of togetherness with other shepherds. Simply put, DAVID DID NOT ENSCONCE HIMSELF WITH THE COMFORT OF FAMILIARITY! He fed the sheep and goats by grazing them in places fraught with the presence of lions and bears.
That courage to lead his sheep and goats among those uncertainties and challenges prepared him for the greater battles with his enemies later in Life.
The initiative shown in shepherding offered David the opportunities to show his courage and valiance. Those victories gave him the provenance to see Goliath’s temple, as the chink to be taken advantage of.
Plus his skill, in Slinging, honed in those shepherding times makes David see what others COULDN’T see & DO.
If effort is nothing, then David wouldn’t have made it as one of the icons of courage, poetic skills and an able King.