It was this [promise] that stood by our ancestors and by us. For it was not only one enemy, (i.e. Pharaoh) that rose up against us to destroy us-in every single generation they rise up against us to destroy us, and the Holy One Blessed be He saves us from their hands.
Just as physical enemies continually rise up to destroy us, so do our inner "foes," our negative impulses, rise up to destroy us every day. And just as G-d protects us from our collective enemies, so does he help us overcome our personal "enemy." As the Talmud teaches:
The negative impulse rises up against us each day to cause us to sin. And were it not for the fact that G-d comes to our aid, we would be unable to overcome our negative impulse.
G-d's promise to Israel is an unconditional one, extending to every generation, in any place and any time and any condition. His promise to protect us from our inner "enemy" is likewise unconditional:
Even if we have lapsed and failed morally, His promise to protect us is not rescinded, and He continues to help us overcome our inner challenges.
There are two times during the year that we utter this prayer: at the conclusion of the Seder and the conclusion of Yom Kippur. The commonality between the two is that both are days when the forces of judgement are not given a voice.
The Seder night, in fact, transcends Yom Kippur in this aspect, since only the Seder night is referred to as a "Night of Watching." Even while one is asleep, one need not fear any danger from the forces of judgement, since it is a night of Divine protection.
At such a time, we certainly have the capacity to declare and bring about that "Next year in Jerusalem!"
According to Siddur Sheloh, one should recite the above declaration three times, since in Jewish tradition a threefold repetition creates permanence, a "chazakah."
Rabbi Schneur Zalman of Liadi, however, did not include this instruction in his Haggadah and so our custom is to recite "Next Year in Jerusalem!" only once.
Rabbi Schneur Zalman's view emphasizes that our power tonight is so great that even without the benefit of a chazakah we can bring about that "Next year in Jerusalem.
From Kehot's Haggadah