Both protons of squaric acid are lost more readily than the proton of acetic acid, even though squaric acid has no aromatic ring to begin with. Why?

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Squaric acid is 3,4-dihydroxycyclobut-3-ene-1,2-dione. Loss of the first proton gives an anion stabilised by resonance and by a strong intramolecular O-H...O hydrogen bond. Loss of the second gives a dianion with four equivalent resonance structures : the two negative charges are shared by all four oxygens, all C-C bonds are equal and all C-O bonds are equal - a quasi-aromatic, highly stabilised system. These very stable conjugate bases explain why both protons are more acidic than acetic acid's proton.

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