Why are noble gases almost non-reactive?

Answer 1

Noble gases have completely filled orbitals / electron energy shells. They are stable and chemically inert (non-reactive). So generally they will not accept / gain / share electrons and they do not form compounds.

Answer 2

Noble gases have a full valence shell of electrons, making them very stable. They do not readily gain, lose, or share electrons with other atoms because doing so would disrupt that stability. This leads to their non-reactive nature.