As anime fans must know, One Punch Man's Saitama has always been portrayed as an overpowered character capable of defeating any opponent with a single punch. However, as revealed by the series, this development also caused him to become very bored.
One of his primary goals when becoming a hero was to challenge himself against strong opponents and reach new heights. Unfortunately, as no one was stronger than him, he effortlessly won all bouts, leaving him with an unfulfilled desire. However, there could be a much deeper meaning behind the Caped Baldy's boredom, not limited to his strength but his spirituality.
Saitama's boredom in One Punch Man could stem from his spirituality

As One Punch Man fans must remember, Saitama decided to become a hero after saving a child despite having no powers. Soon after, he decided to do strength training for three years straight. To everyone's surprise, he became the strongest being, capable of defeating any enemy with a single punch.
Unfortunately, this development did not work in his favor as the Caped Baldy wished to fight strong opponents and grow stronger. However, his unmatched power left him with no real challenge from any supervillain, monster, or criminal, effectively leaving him bored and wanting stronger opponents to approach him.

However, as no such threats approached him, the protagonist has now reached a point where growing stronger would only work against his primary objective. He was already the strongest person around, so growing stronger would only set the bar higher for his possible enemies. Evidently so, the one moment he faced a strong opponent in Garou, his strength adapted to his opponent's.
So, at this point, the A-Rank Hero is not only left with an unfulfilled goal but also struggles to find any genuine motivation to improve. Such a development has resulted in a lack of purpose and a sense of emptiness for the protagonist.

Considering such arguments, it would seem like the Caped Baldy's boredom stems from his exceeding strength. But if we were to take Nietzschean's perspective into account, the reason for the protagonist's despair might be his purposelessness.
Despair isn't simply the loss of hope, but a fundamental feeling of meaninglessness. After Saitama became overpowered, he was indeed bored due to a lack of challenge, but it stemmed from him questioning his existence. Having become overpowered, he had no purpose to become stronger.
Not meeting anyone challenging made him feel valueless. With such purposelessness and lack of value, the protagonist was left with a sense of emptiness.

However, the reason Saitama's despair wasn't so plain as boredom was because there was a potential for growth. According to Philosopher Friedrich Nietzsche, the type of despair the One Punch Man protagonist was going through, while initially painful, could gradually become a catalyst for personal growth.
This meant that, with exposure to the right catalyst, there was potential for Saitama to perceive a new meaning of life, which would not only pull him out of boredom but also help him embrace "life-affirming" values.
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