Why Cu looses 2 electrons to become stable instead of gaining one electron to fill its 4s2 3d10 configuration?
Copper can gain 1 electron to become Cu-.  

Copper has an electron affinity of 1.236 eV, so Cu- is stable to electron loss by 1.236 eV.

See also "The Dissocationi Energy and Heat of Formation
of the Molecule NaCu" Z. Naturforsch. 28a, 316-317 which says:


  But unlike as for NaAg a noticable ionic contribution
  to the bonding is observed for NaCu


Also there is Resonant two-photon ionization spectroscopy of LiCu :


  The unusual shape of this curve is thought to derive from avoided
  crossings between the $\ce{Li^+Cu^-}$ ion pair state and covalent states, with the $\ce{Li^+Cu^-}$ ion pair state
  ultimately correlating to the ground electronic state of the molecule.


So if Cu is to make a compound with an electronegative element, Cu should lose electrons, but with an electropositive element it can gain an electron.