As we learned in the Mishna at the beginning of the perek (50a), the Sages and other members of the community would dance and sing as part of the simhat bet ha-sho’evah (joyous water libation) procession. The Gemara on our daf quotes a tosefta that recorded some of the songs. According to the tosefta, those dancers who grew up devoted to Torah would sing praise for the fact that their youth did not embarrass their old age. The ba’ale teshuvah – those who became committed to keeping the Torah only later in life – sang in praise of their old age, which made up for the sins of their youth. All sang together, praising those who did not sin and encouraging those who did to repent.
It is taught in the Tosefta: They said about Hillel the Elder that when he was rejoicing at the Celebration of the Place of the Drawing of the Water he said this: If I am here, everyone is here; and if I am not here, who is here? In other words, one must consider himself as the one upon whom it is incumbent to fulfill obligations, and he must not rely on others to do so.
Hillel ha-Zaken lived during the Second Temple period and participated in these processions. His unusual comment is interpreted by Rashi to refer to God – i.e., Hillel is speaking on behalf of God, that when His presence is in a given place, then everything is there, but if His presence is missing, then there is nothing of value in that place.
Some explain that Hillel was speaking on behalf of the community at large, and was simply including himself among them.
The Talmud Yerushalmi explains that Hillel’s statement reflected what he saw going on in the crowd. If the people were dancing for their own pleasure and not for the joy of the holiday, then he would sing that if the community were not gathered for the appropriate purpose, then who was there? Does God need crowds of people in the Temple? Does He not have an infinite number of angels who praise him? On the other hand, if Hillel saw that the people were dancing with the proper intent in praise of God, he would sing out that since the community is here, God desires the Jewish People more than anything else and it is as though everything is here.