The shepherd comes home with his lost sheep, and all generous minded neighbours will rejoice with him. This is said in direct contrast to the mean-minded Pharisees.
Why is there joy in heaven? Because God has worked a miracle. It is a miracle that any sinner ever repents. We are so attached to our sin, so caught up in its entanglements, so naturally hostile to God. When Jesus Christ takes a soul home, everyone in heaven rejoices. There is such happiness. But why should heaven rejoice? If I am lost as a sinner, and I come to see my need and I repent of my sin and trust in Jesus Christ, and therefore I am converted and saved, and I love him and walk with him, why should all the population of heaven rejoice? Why should the millions of angels and all the ransom souls who are there already rejoice over one lost sinner who is found? Because they see the scene. They see the enormous gain, and the credit and praise that comes the Lord Jesus Christ. They see what might have happened, and what an immeasurable deliverance has been accomplished. They see how, like the lost sheep, I have been savaged in life by sin and by Satan. I am so ignorant, so aimless, moving to the final day of judgment and account to be condemned forever. And they see the change, that now I am set free. The hearts of those in heaven feel so generous in their good will to those who are saved on earth. I am now forgiven; I have a glorious future; I am a child of the King of heaven. They see the picture, they see the tremendous deliverance and they rejoice over one sinner who repents. And that focus on just one sinner is important. Yes, there are many others – a multitude which no man can number – but each one is a miracle of grace, and will be eternally treasured. Christ encourages us to value each one so highly.