Notice that it is a SHEEP (adult) animal that strays, not a precious, innocent Lamb (immature sheep). The Good Shepherd is risking His Life, and leaving the 99 over an adult sheep who has stupidly wandered off.
There’s hope for all of us, no matter how old, jaded, “experienced”, or stuck in our ways we may be.
True story: one of my daughters came home with a lost lamb she had found. Our home is near a dry creekbed which a shepherd was using for his flock. She was disappointed when I insisted she return the baby to its mother.