This question is answered at the beginning of Luke 15, where we read:

[1] Now the tax collectors and sinners were all drawing near to hear him [Jesus].

[2] And the Pharisees and the scribes murmured, saying, "This man receives

sinners and eats with them." [3] So he told them this parable . . .

Actually, Jesus tells three parables:

 The parable of the lost sheep

 The parable of the lost coin

 The parable of the lost son (or, as we know it, the parable of the prodigal

son)