Jesus taught anyone who was willing to listen to him, including tax collectors who thought Jesus shouldn't be teaching these people. So he told them a story
If a man had 100 sheep and 1 got lost, would he look for it? Wouldn't he leave the 99 others and go look for the 1 missing one until he finds it?
And when he's found it, he'd joyfully carry it home and put it back to its family.
And when he comes back, he'd tell his friends and neighbours "Hey everyone, come together and celebrate. I have found my lost sheep."
It is the same in heaven. For every sinner that returns to god, the 99 others are not celebrated as much as the 1.