Why does boron trifluoride behave as a Lewis acid?

The electronic configuration of boron is $ns^{2} np^{1}$.
It has three electrons in its valence shell.
Thus,it can form only three covalent bonds.
This means that there are only $6$ electrons around boron,and its octet remains incomplete.
When one atom of boron combines with three fluorine atoms,its octet remains incomplete.
Hence,boron trifluoride remains electron-deficient and acts as a Lewis acid.