Krillin did not just wake up one day and decide hey I want to be a strong fighter and train under Master Roshi no this idea came about slowly. Krillin was shorter than most everyone even when he was a kid and became a monk at a young age, even among fellow monks he was picked on because he was smaller and weaker than the others. He heard about the great Master Roshi and decided to find him and train to become stronger. At first he might not have been the strongest kid but he sure was a smart one, the first time he approached Master Roshi to train under him Master Roshi told him no but Krillin knowing that he had a weakness for woman, handed him a magazine (of the pornographic type) and Master Roshi allowed him to train under him. After Master Roshi's training Krillin was one of the strongest fighters on Earth.