Why does Gulliver change his mind about wanting immortaliy?

Gulliver changes his mind about immortality because after celebrating the idea finally realizes what it would be like to grow old and live forever with the infirmities of the aged.

"That the system of living contrived by me, was unreasonable and unjust; because it supposed a perpetuity of youth, health, and vigour, which no man could be so foolish to hope, however extravagant he may be in his wishes. That the question therefore was not, whether a man would choose to be always in the prime of youth, attended with prosperity and health; but how he would pass a perpetual life under all the usual disadvantages which old age brings along with it. For although few men will avow their desires of being immortal, upon such hard conditions, yet in the two kingdoms before mentioned, of Balnibarbi and Japan, he observed that every man desired to put off death some time longer, let it approach ever so late: and he rarely heard of any man who died willingly, except he were incited by the extremity of grief or torture. And he appealed to me, whether in those countries I had travelled, as well as my own, I had not observed the same general disposition."

Society today lives longer because of medical treatments previously unknown. Gulliver's realization fits into our modern society because we live longer lives, but not always better lives. We can treat specific diseases in order to prolong our time on earth, but often extend the time we spend suffering.