He’s always afraid and ready to cry, so why did Zenitsu become a demon slayer?
Zenitsu had to become a demon slayer because he was tricked. Apparently, a woman tricked Zenitsu into taking on a large amount of debt, and a demon slayer-trainer took over the debt. Zenitsu was forced to train as a consequence of his debt being taken over.
Later on, Zenitsu develops far more confidence and begins to enjoy demon slaying.
Zenitsu explains why he had to become a demon slayer
In chapter #20, Tanjiro’s crow instructs him to travel southeast. While on the trail, he hears a commotion and sees Zenitsu grabbing onto a woman’s kimono. Zenitsu is screaming “Help me, marry me!” to the woman repeatedly.
Tanjiro grabs Zenitsu by his clothes and lifts him up. He berates him for starting a ruckus in the middle of the road and harassing a girl who’s clearly not interested.
Zenitsu notices Tanjiro’s Corps uniform and recognizes him from the final selection. Tanjiro says he doesn’t remember, but Zenitsu insists they’ve met and it’s his memory that’s the problem.
The woman slaps Zenitsu, saying that she only tried to help him because he looked sick on the side of the road. Zenitsu interprets that as her being interested, but she says she’s engaged. If he can act like that, she says, then he’s healthy enough to go home.
Tanjiro asks how someone so weak and pitiful could become a demon slayer. Zenitsu explains that a woman tricked him and left him with a ton of debt. His trainer was an old man who took on that debt, and Zenitsu’ life became “an endless hell of training.”
He claims he thought he wouldn’t survive the final selection. Yet, Zenitsu was able to pass the final selection, somehow.
So that’s the answer. Zenitsu had to become a demon slayer because he was tricked. A woman gave Zenitsu a ton of debt, and a trainer took on Zenitsu’s debt as long as he became a swordsman.