Why does sodium usually form a Na+ ion?

Sodium typically forms a ${\text{Na}}^{+}$ ion because it is an alkali metal with a single valence electron in its outermost 3s orbital.1

In chemical reactions, it is energetically favorable for sodium to lose this loosely bound valence electron to achieve a stable, closed-shell noble gas configuration, which is identical to that of neon. While removing this first electron requires an input of ionization energy, the process is facilitated during the formation of ionic compounds by the release of lattice energy as ions pack into a crystalline structure.123

Removing a second electron from sodium is not typical because it would require stripping an electron from the atom's inner core. Core electrons are held much more strongly by the nucleus than valence electrons, making the energy cost for such a removal prohibitively high for standard chemical reactions.2