There's another part of that story that people usually forget. The whole situation was compared to a shepherd losing a sheep. That shepherd, it stated, would leave the rest of his flock to go search for that wayward sheep. And when he found it, would celebrate its return. So the whole thing is just a metaphor (allegory?) for someone losing their faith, then coming back to the church. And when someone does return, you realize how much they were missed and celebrate. It's more of a you don't know what you have until it's gone idea.
At least that's the interpretation I remember from religion class. Personally, I think the other sheep in the flock would have run off somewhere else while the shepherd abandoned them to go find the one stupid sheep, but that's just me.
Besides, who wouldn't celebrate their son returning after they thought he was gone for good?