A Universal Altar

Water symbolizes Your might...It reassures, with its drops, those in whom was blown the breath of life.
— Mussaf Prayer Service

A sukka is such a strange little makeshift building. It's usually such a temporary structure that it cannot be made large enough to accommodate many guests, even though we have a tradition of inviting guests and strangers into our sukkot. This itself is a lesson in hospitality. It doesn't matter what our homes look like, it matters how hospitable we are. But some sukkot, as discussed in the mishna, are so small that they could only fit the majority of one person in them. Sometimes we simply cannot build a big sukka so we kvetch out a little space to call our own. 
In contrast to the small size of a sukka, consider another less well-known law of sukkot in the ancient days of the Temple. Seventy sacrifices were offered on Sukkot, corresponding to the 70 nations of the world - a number widely assumed to embrace the totality of the ancient world. Each day, the animals offered lessened in number, exactly the opposite of the way one lights Hanukah candles. The Sefer Ha-Hinukh, a medieval compendium of the commandments, suggests that in the merit of this commandment, the enmity of the nations will lessen against Israel. If the sacrifices are offered in the name of foreign nations and they lessen in number over time, then, so too will any bad feelings towards Israel gradually be reduced. Sukkot demands that we create small, alternative homes for ourselves and large overt gestures to the outside world.
The Talmud in tractate Sukkot is a little more radical in its assessment of these sacrifices. Rabbi Yohanan, a famous talmudic sage, bemoans the Temple's destruction in an astonishing way: "Woe to the Gentiles who lost so much without realizing that they lost anything at all! When the Temple was standing, the altar gained penitence for them, and now, who will atone on their behalf?" Rabbi Yohanan not only saw the universality of Sukkot and the altar, but felt the pain of the other at losing this opportunity. Everyone must receive the privilege of atonement.
Rabbi Yohanan's statement, as merciful as it sounds, also questions the possibility of non-Jews receiving atonement through other agencies. He assumes that without the Temple, they have no other means. This may be an implicit criticism of other religions. Perhaps for this reason does Rashi take another interpretive stance.  In his commentary on the Talmud, Rashi wrote that the 70 sacrifices offered on Sukkot correspond to the 70 nations of the world who are judged, as is Israel, at this season for the year's rainfall.  Rashi is trying to explain what aspect of mercy we are seeking from God with all these sacrifices and identifying a universal concern that we all share.
Rain, rather than atonement, is our primary concern. Rashi most likely extracted his explanation from Zechariah 14. There, a strong and definitive case was made against nations who did not take advantage of the Temple's services to the broader community on Sukkot:

"All who survive of all of those nations that came up against Jerusalem shall make a pilgrimage year by year to bow low to the King of Hosts and to observe the Feast of Booths. Any of the earth's communities that does not make the pilgrimage to Jerusalem to bow low to the King of Hosts shall receive no rain."

These verses, harsh as they may sound, advise the whole world to concern itself as an organic, interdependent entity united by that which is pressing for all of humanity. Everyone needs rain, and no one is exempt from praying for it. Although this pre-dates our worry over the ecology by millennia, it reflects many of the same concerns.
The prayer for rain, uttered during the Musaf or additional service of Shmini Atzeret, touches us with its urgency and its poetry. There, too, despite the many expressions of a particularistic faith - from patriarchs associated with water to the high priest's water ablutions on Yom Kippur - there is an appeal to the basic needs of us all: "Water symbolizes Your might...It reassures, with its drops, those in whom was blown the breath of life."
Images of water and breath, Jew and non-Jew, home and universe, work together in concert as nature meets the divine. Sukkot allows us the dual benefit of living introvertedly and praying extrovertedly.
Shabbat Shalom and Happy Sukkot!