Why noble gases are non-reactive?
Noble gases have a full valence shell, so they do not react with other elements.
As you may or may not know, atoms of elements take and give electrons in order to form bonds and, therefore, compounds. However, noble gases have a full valence shell (8 electrons in the valence shell).
Normally, in an atomic reaction, there is an instability in the atoms' valence shells. For example, oxygen has only 6 valence electrons. In order for an atom to be completely stable it needs to have 8 valence electrons. So, two hydrogen atoms with one electron each bond with the oxygen atom, creating a stabilization.
However, in the case of noble gases, their atoms already have a full valence shell, so there is no instability and no need to form bonds with other elements. In fact, noble gases got their name from their inactivity with other elements.