Warning: contains spoilers for Superman: Son of Kal-El #6!
Both Superman and his son share incredible physical strength, and are arguably two of the strongest superheroes in the DC Universe - but that strength comes with a terrible cost. Both Clark Kent and Jonathan Kent live among humans (and Jonathan himself is half-human), and must control themselves lest they reveal their super-strength. Superman: Son of Kal-El #6 is a perfect example of the extremes Superman must put up with to exist in a fragile world.
In the Superman: Son of Kal-El series, Clark Kent has left Earth, and his son Jonathan has officially taken up his father's mantle as the planet's new Superman. Jonathan has all his father's powers but lacks his experience, and thus has quite a lot to learn in the ways of superheroics within the DC Universe. Nevertheless, he tackles such current-day subjects as climate change, immigration, inequality, and even modern-day dating and relationships.
In the case of the last issue, Superman has just began a relationship with his new boyfriend Jay Nakamura. After Nakamura saved Superman's life in the previous issue, Jon owes Jay, and thus approaches his apartment in the heart of Metropolis. Jay sees Superman knock on his window, and astonished, opens it to find Superman flying at eye-level. "You can knock down a mountain. How do you knock on a window?" asks Jay. Superman only responds "Very, very carefully."
Jonathan Kent is modest as befits his nature, but he's not wrong; Superman must exhibit extreme control of his own body to prevent the world around him from crumbling - or innocent people getting hurt in the crossfire of battles. Superman canonically pulls almost every punch he throws, just in case his strength kills someone (even a criminal). The Justice League Unlimited episode "Destroyer" contains a monologue during a battle with Darkseid that underlines the first Superman's frustration. He mentions he feels as if he lives in a world made out of cardboard, taking care to always hold back his true strength, and "...never allowing myself to lose control - even for a moment - or someone could die."
Superman has spent a lifetime of maintaining that control - but the same cannot be said for his son. Without the years of experience of his father, Jonathan Kent must consciously move around with incredible caution. Superman's true strength is the ability to control that strength - and the people around him depend on it.