“If you think you are the best, you will not become the best you can be. “
Roshi later explains that he did this because Goku at 12 years old was probably already at the top of what martial arts had to offer at the time, and he wanted Goku to still believe there were people out there stronger then him so that he would still have the drive and motivation to continue to train to reach his full potential. He feared that by realising he was the best, Goku would become unmotivated, and thus stop growing.
We can learn a lot from this sentiment from two great masters. They both knew that no matter how much passion and love Goku had for martial arts, if he believed he was the best he would not work as hard, and thus would not be able to reach his true potential. This is something I actually talked about briefly in my article about Ace’s on a team. The Ace’s who believe themselves to be the best become lazy (I used Ovechkin as an example). This is something to keep in mind in your own life, and for anything you are passionate about. If you think you are the best, you will not become the best you can be. Maybe you are the best in your class, town, city, country, but remember somewhere, someone is better than you, or working harder to become better than you. Use this as motivation to keep pushing, trying new things and expanding your knowledge to truly reach your potential. If you can do this I think you will even surprise yourself how far you will get.