Why Does Krypton Hurt Superman?

It could be that Kryptonite affects Superman by interfering with his cells and their ability to metabolize solar radiation. Thus, it makes him weaker due to the inability of his cells to process the radiation necessary to give him his superpowers!

Why does Krypton weaken Superman?

The prevailing theory in geekdom is that kryptonite can kill Superman because it disrupts his solar panel-like energy absorption. Without energy from the Sun, he weakens and eventually dies.

Does Kryptonite hurt on Krypton?

In almost all known continuities it is Green Kryptonite which has proven to be lethal to Kryptonians who have been exposed to its radiation. Green Kryptonite was lethal to any Kryptonian from the same universe the Kryptonite was derived from.