How each opponent Goku fought indirectly helped him get stronger

Yeah, we all Know Goku is the strongest warrior who saved the earth multiple times. But we should also not forget that his strength is the result of the constant challenge he faced.

For Goku fighting was just something fun, he fight because he love it but it all changed when he meet the Red Ribon. The Red Ribon was a powerful army destroying anything on their path and become Goku's worst enemy in early DB. they were a menace to not only Goku but to also his friends and wanted to kill him sending powerful fighters one after another to get the job done. This forced Goku to take the matter personal and destroy the Red Ribon by invading their Head Quarter alone. After that Goku only keeps training harder knowing he needs to be strong enough to protect his friends. From there fighting was not just for fun anymore for Goku but a necessity to protect the world and his loved ones.

This fact become even more apparent after Goku's friends were killed by king Picollo and the world was close to extinction. Goku seeks help from the Lord of the Tower to beat Piccolo. While Goku won the battle it wasn't over cause Piccolo gave birth to his son to revenge on him. This pushed Goku again to train himself more harder with the All-powerful also known as the creator of the Dragon Ball.

But years after his victory against Piccolo Junior, Goku seems to start taking his training less serous. Him being now the strongest warrior on earth made him and also a father stop training constantly. This ends up costing him his life against his big brother Raditz. After that Goku again become aware of the danger thanks to Radditz and took his training seriously with Kaio. From there Goku only keeps getting stronger and stronger knowing full well that if he isn't strong enough it will be over.

Many years passed again and Goku is now facing the cyborg this time Goku is against an opponent that even he can't beat and only someone else can, his son Ghan. this reality made Goku pass the torch to Gohan, not only to beat Cell but to protect the earth in his place.

Still, even after he's death, Goku keeps training and managed to achieve a level never achieved by any Saiyen the SSJ3 form that only him can use which was necessary late to overcome Buu.