The Parable of the Lost Sheep
The scribes and Pharisees complain that Jesus receives sinners and eats with them. Jesus says if you have 100 sheep and one is lost don't you leave the 99 in open country and go after the 1. In the same way heaven rejoices when 1 sinner repents more than when 99 righteous need no repentance.
Makes sense, maybe the lesson is to use your resources where they are most needed. Although I imagine most use it as an order to proselytize.
The Parable of the Lost Coin
If you have 10 coins and lose one you won't you diligently look for it until you find it? When you find it you will rejoice, this is like heaven when a sinner comes back.
The Parable of the Prodigal Son
A guy has 2 sons, one asks for his share of his fathers property so he gets it. He then leaves town and squanders all of his money. There is a famine and he has a hard life for a while. He then decides he should return to his father as a servant. When he gets there his father treats him well. He throws a big party. The other brother is upset because he has always been with his father and hasn't gotten this treatment, the dad says it is a day to rejoice because his brother is effectively back from the dead.
Same point as above it seems