[rentj]

I have been taught that in general, atoms need energy to remove one electron from it while it releases energy when capturing an electron. Then, why does Na atom prefers being Na+ instead of Na-, since the enthalpy of electron capture for sodium is negative (exothermic), and the enthalpy of ionization for sodium is positive (endothermic). Generally, if the enthalpy is negative, the reaction would be spontaneous. Thus, why does Na atom prefer consuming energy (ionization) instead of capturing an electron (electron affinity) which is more spontaneous? Thank you.

[Borek]

I have been taught that in general, atoms need energy to remove one electron from it while it releases energy when capturing an electron.

That's for an isolated atom, not interacting with its surroundings. Put Na^- between other atoms/molecules, and it will happily get rid of the extra electrons, forcing other things to keep them.