Among the commentaries there is some disagreement as to whether the first offering of wine was in kindness with a wine mixed with frankincense or other herb (in relation to Proverbs 31:6-7 which advised to give strong drink to the condemned), or that both wines were the simply the cheap common Roman sour wine often drank by soldiers. In either case, it does not matter as the point is that it was offered as a mercy to condemned persons to alleviate their sufferings. In the first instance Jesus refused it, in the second, it expressed his thirst and he drank it.
There is the subject of prophecy fulfilment in this story and practical reasons why the prophecies were fulfilled in the way they were. Although it may be agued that Jesus had an eye to the fulfilling of scripture and actually spoke ‘I am thirsty’ to fulfil it, it seems that without also a seeming natural explanation how the events unfolded into the fulfilment, a solid exegesis is not complete. To answer the question, therefore we must answer how this fulfilled scripture and why it was natural so to do given the circumstances
First, how his expression of thirst and his drink of sour wine fulfilled the scripture we see in these two Psalms, one expressing his thirst and the other indicating the bitterness of the drink, possibly symbolic of the sin which he had died for:
My mouth is dried up like a potsherd,
and my tongue sticks to the roof of my mouth;
you lay me in the dust of death.
Dogs surround me,
a pack of villains encircles me;
they pierce my hands and my feet.
All my bones are on display;
people stare and gloat over me.
They divide my clothes among them
and cast lots for my garment. (NIV Psalms 22:15-18)
They put gall in my food and gave me vinegar for my thirst. (NIV Psalms 69:21)
Second, why his denial of the first drink and his receiving it and even asking for it on the second occasion seems natural in the fulfilment of scripture, is in my view very absorbing. Some commentators just superficially count it as Jesus knowing it would not match the prophecy exactly as the first drink was offered before the scene of the Psalm is fully realized. Other commentaries do not think Jesus was so directly orchestrating events to fulfil scripture but that there is a definite natural reason. Accordingly it seems that what is really going on is that Jesus wanted to be fully alert in facing the penalty of God for sin and in suffering for it. He did not want to be dulled before facing his greatest work and effort. However, after he had finished the work of atonement, he was free to concern himself with the mere needs of his body. Therefore his only complaint on the cross, saying ‘I am thirsty’ signifies that his atonement work was fully finished and he became conscious of his own needs as expressed in the Psalms. No longer was he flooded by the ocean of sin unable to speak or necessarily even consciously think, as he was baptised in a baptism we can never fathom or begin to enter into the mystery of. Once this was complete he attended to his own needs with nothing left to do and he said ‘It was finished’ and he died.
Conclusion: It would seem that the change in his attitude about drinking sour wine indicates that the spiritual and difficult part of the atonement was fully completed, so Jesus could care for his own needs. This completion of the atonement ending in a conscious thirst, with cruel sinners surrounding him, fulfilled the scriptures.