Why did Zenitsu become a demon slayer when he was always scared and ready to cry?
Because he had been deceived, Zenitsu had no choice but to become a demon slayer. Zenitsu was tricked into taking on a huge debt, which the trainer of the murderous demons then took. As a result of his debt confiscation, Zenitsu was forced to train.
Zenitsu gains confidence over time and learns to love demon slayer.
Zenitsu explains why he had to become a demon slayer
Tanjiro’s crow tells him to fly south-east in chapter # 20. He feels disturbed and sees Zenitsu holding a woman’s kimono while on the way. Zenitsu kept yelling at the girl, “Help me, get married!”
Tanjiro snatches Zenitsu’s clothes and pulls him to his feet. He reprimands her for making noise in the middle of the street and abusing a woman who doesn’t care.
Zenitsu has known Tanjiro since the last election because of his Corps uniforms. Tanjiro says he does not remember meeting Zenitsu, but Zenitsu swears that they once met and that his memory is a problem.
Zenitsu was slapped by the woman, who said she was trying to help him as he was sick on the side of the road. Zenitsu takes this to indicate that she is interested but says she is engaged. He says if he can behave like that, he is well enough to go home.
Tanjiro wonders how a weak and miserable person can be a demon slayer. Zenitsu reveals that he was deceived by a woman and is now in debt. Zenitsu’s coach, an old man, assumed this responsibility, and his life became a “perpetual torture of training.”
He believes he did not think he could reach the final round. On the other hand Zenitsu managed to win the last selection.
So you have it. Because he had been deceived, Zenitsu had no choice but to become a demon slayer. Zenitsu owed a young woman a large sum of money, and Zenitsu’s obligation was taken by the trainer as long as he was a swordsman.