From Kollel Iyun Hadaf’s Insights to the Daf (dafyomi.co.il):
The SHEM MI’SHMUEL (Sukos 5672) explains that heretics, who do not acknowledge that Olam ha’Ba exists, believe that the purpose of life is to achieve joy and happiness. The two heretics mentioned in the Gemara disputed which type of joy is greater — the type known as “Simchah,” which refers to a continual, constantly-increasing feeling of happiness, or “Sason,” which refers to sudden, spontaneous joy, the type one feels when stimulated by a surprise. (The VILNA GA’ON says that “Simchah” is an inner joy, while “Sason” is an outward expression of joy. See YA’IR OR, “Gil.”) Each heretic tried to prove that the other form of joy is dispensable.
When Sason debated with Rebbi Avahu, he said that in the World to Come, Rebbi Avahu will fill up water for him, as it says, “u’She’avtem Mayim b’Sason,” which the heretic interpreted to mean, “You will fill up water for Sason.” The heretic asserted that even in the World to Come man has no more noble purpose than the attainment of joy.
Rebbi Avahu responded that the attainment of happiness is not the ultimate purpose in life. Rather, joy is a means to an end; it is a vehicle to help a person serve Hash-m. “U’She’avtem Mayim b’Sason” means that Sason, joy, will help a person draw Torah (which is compared to water) and attain Ru’ach ha’Kodesh (see Tosfos 50b, DH Chad). Joy enables a person to fulfill the ultimate purpose of life — to serve Hash-m and achieve closeness to Him through learning Torah and doing Mitzvos.