As a superhero, Saitama is different. Most heroes are who they are because they have a burning desire to make a change in the world and keep people safe from the endless dangers of their fictional world. But Saitama? He's just a hero for fun. Saving the world is just a hobby to him.
Episode one's Saitama went through quite a transformation to become who he was in episode twelve. The man was miserable, to say in the least. He explained how he was experiencing a lack of emotions: happyness, sadness, anger, panic and even the exhleration he used to get from fighting. Saitama felt nothing anymore. Prior to becoming a superhero, he was worse off. His eyes were dull and lifeless; Saitama didn't care at all if he died because, well, being a businessman wasn't his thing. However, he found that being a superhero had quickly become a bore, too. After unofficially achieving the title of, "The Strongest Man" he hadn't set out for something to make his emotions return, but he'd found what he needed: A friend.
Here, Saitama's friend, Genos, is offering to get him a new suit while they eat lunch together.
To Saitama, Genos was what he needed, although a nuisance at first. But soon, the two characters became close and what was acting, became clear to be actual friendship shining through their hit series: One Punch Man. Although the series has ended, a second season hasn't been decided, but the manga is still ongoing.