Albert Barnes on Matthew 18:12-14

To show still farther the reason why we should not despise them, he introduced a parable showing the joy felt when a thing lost is found. Man rejoices over the recovery of one of his flock that had wandered, more than over all that remained. So God rejoices that man is restored, seeks his salvation, and wills that not one thus found should perish. If God thus loves and preserves the redeemed, then surely man should not despise them, See this passage farther explained in Lk 15:4-10.

(i) "if a man" Lk 15:4