Adam Clarke on John 19:28

I thirst - The scripture that referred to his drinking the vinegar is Psa 69:21. The fatigue which he had undergone, the grief he had felt, the heat of the day, and the loss of blood, were the natural causes of this thirst. This he would have borne without complaint; but he wished to give them the fullest proof of his being the Messiah, by distinctly marking how every thing relative to the Messiah, which had been written in the prophets, had its complete fulfillment in him.