Why noble gases are non-reactive?
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.