Exploration Vs Exploitation

In his 2010 letter to shareholders, Warren Buffett tells a joke —

Customer: Thanks for putting me in XYZ stock at 5. I hear it’s up to 18.
Broker: Yes, and that’s just the beginning. In fact, the company is doing so well now, that it’s an even better buy at 18 than it was when you made your purchase.
Customer: Damn, I knew I should have waited.

Reading this joke the first time, I laughed at broker’s convoluted argument and the stupidity of the customer. How can a stock be better buy at 18 than it was at 5?