He fights so he doesn't have to work.
It's a dystopian future where the world is dominated by the rule of work. Those who commit their lives to their jobs and become brainwashed drones are treasured. Those who would be parasites, living off the hard work and resources of the working population, are hunted down and forced to find their place in the labour force. This is the world of Koukidou Mushoku: Neetenberg (高機動無職ニーテンベルグ or "High Mobility Unemployed: Neetenberg")