Answer: Their viewpoints depended on their respective backgrounds.
Step-by-step explanation:
Alexander and the gymnosophist laugh at each other because for each one the other seemed as embarked on a worthless pursuit.
For the gymnosophist, it was ridiculous for a man to conquer the entire world. For Alexander, someone contemplating nothingness seemed as a waste of life.
Their viewpoints depended on their respective backgrounds. Alexander´s background makes him think that we only live once and it´s important to make it count, as only heroes who accomplish magnificent achievements win glory for their memory and immortality in Elysium. On the other hand, the gymnosophist believed hehad many lives and deaths, and even the greatest efforts might not amount to much, as things are constantly changing on their own.