Ionization energy is the energy needed to remove an electron from an atom or ion to form a positive ion.
The first ionization energy is the energy needed to remove one electron from an atom (atom are neutral species) to form a positive ion. This electron is the electron with the higher energy level (it is from the valence shell, of course). It is the electron easiest to release.
The second ionization energy is the energy needed to remove a second electron from the atom. It implies to remove an electron from the positive ion that was formed after removing the first electron from the neutral atom. It is harder to release an electron from a positive ion than from a neutral atom.
So, it is the higher ratio of the protons to electrons which make succesive ionization energies to increase.
In conclusion: The ionization energy increases because the ratio of the protons to electrons increases.
