The hero will save the people in need, Soro trusted this when he was small. When he was being bullied, he waited for the hero to save him, but hero didn't come; when his village was being attacked by Noxus army, he waited for the Demacia hero to save him, but only his sister and he were saved, his parents and most of the villagers were killed. When his sister was caught by a Zaun crazy scientist, he tried to fight with him but he lost, and still no hero came to help him.
Soro was disappointed, and he heard the story of the queen of the haunted ground. He jumped into the haunted ground, and he faced the phantom created by Gae Bolg. In the phantom, he found that all the misfortune in his life was his fault. He was bullied because he didn't try to be strong, his parents were killed because he was too weak, and his sister was caught because he didn't face his weakness even after so many things had happened. In the phantom he saw the regret of the people killed by Gae Bolg, they all begged all of them before they were killed. Soro noticed that he didn't deserve for sympathy, he shouldn't regret because he was still alive.
He cried out loudly after he overcame the phantom, which woke the queen of the haunted ground up. Soro made the last begging in his life to Scathach, “Teach me how to become a hero”.
He became the pupil of Scathach, he was taught the skill of battle, but he does not have great talent. Scathach focused on training his body strength and will power, which required no talent and just needed to bear the great pain.
After several years, his master said it is time for them to go to the field of justice. But before that, he went to Zaun and killed the scientist who caught his sister, and he knew that his sister was saved by Janna, the storm’s fury. He was glad about his sister was saved by a hero, Zaun sent their army to catch Soro, but Soro left in a laugh, none of Zaun’s weapons can hurt him. And the world started to know about Soro because of his unbreakable strength, and called him “The Man of Steel”.
“Do not beg, learn to fight.” - Soro
Link to Scathach: http://leagueoflegends.wikia.com/wiki/User_blog:Walter5ck/Queen_of_the_Haunted_Ground_-_Scathach