We know that Bnei Yisroel were on a low spiritual level after the many years of bondage in Mitzrayim, and Hashem had to bestow upon them an abundance of holiness in order for them to reach a level where they would be eligible to be redeemed. Through this act of benevolence, Bnei Yisroel climbed many rungs on the spiritual ladder in an extremely brief amount of time. They reached the pinnacle in their spiritual ascent on the night before they were released from bondage.
Nevertheless, the Ramchal tells us that in the forty nine days between Yetzias Mitzrayim and Shavuos, Bnei Yisroel extracted themselves from the forty ninth level of impurity to which they had sunk, and ascended to the forty ninth level of holiness. Rav Wolbe asks that if they had reached the spiritual zenith when they left Mitzrayim, why did they have to work for seven weeks to climb to a plateau on which they already stood. The answer is that the tremendous level of greatness experienced at Yetzias Mitzrayim disappeared the following day, and they needed to reestablish that level, but this time through their own efforts. Hashem dispenses a flash of inspiration, and then we are expected to revive that sensation through our avodas Hashem.
In a similar vein, Chazal tell us that at the time of Creation the light was so great that Adam could see from one side of the world to the other. However, because of the wicked that would not be worthy of this light, Hashem "hid" it and set it aside for the righteous. If Hashem knew that He would hide it, why did He create it in the first place? Additionally, we know that a child in the womb experiences this otherworldly light, and is taught the entire Torah, only to forget it all when he is born. Again we must ask, if a child forgets everything when he is born, what is the purpose of placing him in a spiritual utopia in his mother's womb? Both these questions can be answered with the above concept. Hashem gives a flash of inspiration which leaves an indelible impression, thereby enabling us to achieve the goal toward which we must strive.
This is an idea which rings true in each of our lives. Many people feel a surge of inspiration - be it after the Seder night or any other inspiring event. However, those feelings wane after a few days and they might feel that they are back to square one. In reality, what happened is that Hashem bestowed upon them a spiritual revelation, and then retracted it to allow them to work their way up to that level on their own accord. When the flash of inspiration disappears, there is no reason to feel despondent; rather, it should be the impetus for our avodah and a means of becoming closer to Hashem.