There was one lost sheep — we don't know how or why they are lost or whose fault it is. They just need to be found. God desires all people to be saved (2 Pet 3:9)
The shepherd left the 99 to find one — he is scared and worried, but hope overcomes all these fears. The shepherd understood the value of one. Who will go? We will and we need to have compassion for the lost. Are we willing to be uncomfortable to teach the lost? When will we feel what we need to feel to reach out to our loved ones who are lost?
Oh the joy in heaven and in earth when one lost person is found! — they joy of the shepherd when he finds his sheep. It's like the joy of Mary and Joseph when they found Jesus in the temple (Luke 2:41–52). Think on their desperation as they sought out their son. Where is our desperation? We come and go in the world without a thought of the lost people in it.