The shepherd wasn't satisfied with 99 percent of his sheep safe within the fold. Nor did the shepherd assume that it would come back on its own. That sheep was indifferent to its lost condition and needed to be found. So the shepherd took the initiative and went searching for it. His friends and neighbors who celebrated with him must have felt that to find a lost sheep was one of the most joyous and worthwhile endeavors in the world.
Christians have an obligation to seek out the lost.
Then, like the shepherd in Jesus' parable, we too would be zealous in seeking the lost sheep. To be your brother's keeper means that you will be his seeker.