Elsewhere Jesus presumes that all people are naturally evil and only God, good. In Matthew the prophesy of Ezekiel 34 is undoubtedly in the mind of Jesus as he relates this extended similitude to his disciples. Jesus gently warns his disciples not to be that kind of leader, but to follow his example as he follows the Father's. The teaching in Luke conveys a different nuance of emphasis. In Luke's Parable of the Lost Sheep, Jesus implies that God is like a Good Shepherd and that surely lost persons are more precious than lost sheep.
None is too lost or too insignificant to be sought.
Perhaps we should be more tolerant of the returned straying lambs, injured by their wandering far from the fold. Perhaps, if we did, we would not be guilty of driving away sheep for which the Great Shepherd gave his life.