In Shemot 32:20 Moshe tells the people: "You have sinned a great sin; now I will go up to God, perhaps I will [be able to] atone for your sin." This, in particular the "perhaps", should be quite disconcerting. Yet there is no hint that the people regret their deeds or begin to do teshuvah.
But when Moshe comes down from the mountain, the people react quite differently. God tells Moshe to take the people to the fruitful, promised land, from which God will expel the inhabitants so that Israel can inherit it. However, God will not be "in their midst" as they travel - for their own protection, lest they sin again and incur destruction. This seems like a pretty amazing deal for a people who had just reverted to idolatry so soon after receiving the Torah. Yet the people are much more distressed by it than by the previous announcement, that they "might" receive atonement. Now, "the people heard this evil message, and they mourned, and no person put on his ornaments." (33:4) (The message is called "evil" to indicate that the people saw it as evil.) Why do they mourn now, when they should have mourned much more beforehand?
This oddity can be explained by studying the people's behavior over a long period, from the giving of the Torah until the Mishkan is built. Nowadays we all know people who constantly pursue "spirituality" in their lives, whether it takes them to chassidut, kabbalah, Buddhism, Apache rain dances, hallucinogenic drugs, or all of the above simultaneously. This is a desire that most of us just don't have. While it means that our religious lives can be stale and passive by comparison, it also saves us from the excesses and immorality often encountered when sampling exotic doctrines indiscriminately. I think the entire Jewish people possessed such a intense thirst for "spirituality", throughout the period they were at Mount Sinai.
On several occasions this led to great accomplishments. When offered the Torah in parshat Yitro, they enthusiastically accepted. And later on, when offered the chance to contribute to building the Mishkan, they brought so many donations that they had to be told to stop. Unfortunately, when no such immediate spiritual opportunities were available, they were unable to restrain themselves. When Moshe delayed to come down from the mountain, they could not stop themselves from making and worshipping a calf. If they could not have legitimate spiritual experiences, then illegitimate ones would have to do.
Thus, rather than being an inexplicable oscillation between extremes, the people's behavior stems from a single consistent behavioral trait. The acceptance of the Torah, the golden calf, the people's distress at hearing God will not travel with them, and their eagerness to contribute to the Mishkan - all result from a thirst for spirituality, unbalanced by a sense of boundaries or patience for when spiritual experiences are unavailable.
It is perhaps in reaction to this behavioral trait that the book of Vayikra (which follows the building of the Mishkan) enacts boundaries and restrictions in every aspect of life. The indiscriminate spiritual urge may still be present, but when it cannot be indulged it is not dangerous.
2. Its source
Where did this excessive, unprecedented spirituality come from? My guess is that it results from the unprecedented miracles the people experienced in and while leaving Egypt. This experience habituated them to regular, spectacular Divine revelations. Even people who normally had little spiritual drive would likely become interested when confronted with such overt miracles.
This hypothesis may be tested by examining the people's behavior between leaving Egypt and reaching Sinai. In this period, there were two occasions when the people challenged Moshe and, effectively, God. Do these complaints disprove the thesis that in this period the people desired constant closeness with God? Let us see.
The first of these complaints revolved around the people's lack of food, the other around lack of water. As is explained here, the complaint about water reflected a fear that God had abandoned the Israelites and Moshe was simply leading under his own powers. Lack of water is the immediate cause of the complain, but the people's desire for a constant connection with God is evident throughout.
The same seems to be true of the food complaint. There the people complain: "If only we had died by God's hand in the land of Egypt... for you have brought us into this desert, to kill this whole assembly by hunger." The people's attachment to God is so strong, they say, that even if they have to die, better that it happen through Divine intervention than through the normal laws of nature. Moshe tells them that "your complaint is not against us [me and Aharon], but against God" (16:8), indicating that until then, the people did not realize they were implicitly rejecting God.
The nature of these complaints seems to confirm that the people's spiritual desire did not wane between the Exodus and the events at Sinai. If anything, the people complain that God is absent rather than that God is doing something wrong.
3. An implication
This theory could have significant implications regarding other events that happened around the same time.
For example, it is commonly said that women were not involved in chet haegel. One possible source for this idea is the fact that women took the lead in contributing to the Mishkan (see Shemot 35:22, 38:8, and commentaries). If they were so heavily invested in this good deed, then how could they have supported the evil deed which preceded it? But according to my theory, this argument should lead to the opposite conclusion. The golden calf and contributions to the Mishkan were morally opposite, but they derived from the same spiritual motivation. Perhaps women possessed a greater spiritual urge and thus contributed more to both calf and Mishkan. This would match the contemporary experience of Reform Judaism and other liberal movements, that women are much more interested in religious involvement than men.
(For the record, I think all arguments along the lines of "men/women are better than women/men because at some historical moment they did/didn't do something good/bad" are silly.)
4. The future
The people's spiritual interest seems to have been constant throughout the first year or so after leaving Egypt. But it did not last forever. By the 11th chapter of Bamidbar, when another set of complaints and rebellions begins during the journey to Israel, the people's nature seems to have changed. They still have a sense of expectation, but now their expectations are material rather than spiritual. They now behave like spoiled little children, demanding to eat delicacies and refusing to accept the challenge of conquering.
Spiritual experiences are opportunities to point your life in a new and better direction. But if the opportunities are missed and the excitement passes without any lasting changes being made, then you will likely still be changed, but in a way you did not intend. It is impossible to stand still in life: one is always moving either forwards or backwards. If you do not have the courage to move forward then you will slide backwards, whether you really intend it or not.