In Dr. R. Alan Culpepper's commentary on Luke, he ends the first two parables in Luke 15 with the following observation:

In both parables, rejoicing calls for celebration, and the note of celebration may be exaggerated to emphasize the point. Neither sheep nor coins can repent, but the parable aims not at calling the "sinners" to repentance but at calling the "righteous" to join the celebration. Whether one will join the celebration is all-important because it reveals whether one's relationships are based on merit or mercy. Those who find God's mercy offensive cannot celebrate with the angels when a sinner repents. Thus they exclude themselves from God's grace.

I think that this is a very important question for churches today. More than one congregational member has not rejoiced over the influx of new members in the congregation. The non-rejoicing criticism seems especially prevalent when the "found" are somehow different from the mainstream members.