Why do noble gases almost never form chemical bonds?
They already have full valence shells.
Chemical bonds, whether they're ionic or covalent, involve the sharing of electrons so that atoms can have a full "valence shell", which is its outermost layer of electrons. Each new shell is represented by a row on the periodic table.
Noble gases, on the far right, already have full electron shells. Thus, they achieve a low energy level and are content without having to bond.
In contrast, halogens, the second furthest group on the right, want to gain one electron to attain a full valence shell, so they are very reactive.