Those who offer the korban Pesach on the 14th of Iyar follow the same basic procedure as those who brought it one month earlier, on the First Passover. There are, however, several legal and procedural distinctions between the two Passovers, the most important of which concerns the prohibition against leaven.
On the First Passover, leaven is strictly forbidden from noon of the 14th of Nissan (the earliest time at which the korban Pesach can be brought) until the conclusion of the festival; throughout this period, no leaven may be eaten, used in any way, or even be present in one's domain.
On the Second Passover, however, this prohibition does not apply. While the korban Pesach is to be eaten with matzah, the unleavened bread, there is no mandated exclusion of leaven; in the words of the Talmud, "leaven and matzah coexist in one's home."
Leaven is dough that has "risen" - flour and water that have come in contact and have been allowed to ferment, with the effect that the mixture has bloated and exaggerated its mass. Leaven is thus the symbol of egotism and pride, a "leavened" soul is one in whom the ferment of self-importance has caused him to lose sight of his true place in G‑d's world, with the result that he recognizes only his bloated self and its inflated wants.
This explains why the prohibition against leaven on Passover is so severe and uncompromising: in no other instance does the Torah not only forbid the consumption of, or derivation of benefit from, even the smallest quantity of a substance, but also its very existence in our possession.
But egotism and pride is not just another undesirable trait - it is the source of all evil in the heart of man. Every sin and vice originates in an assertion of ego - with the sense that the self is supreme and that its needs and desires take precedence to all else. Thus, in his Laws of Human Character, Maimonides advises that in all traits a person should pursue the "Golden Mean," with a single exception: pride. Pride must be vanquished utterly.
This is not to say that there is nothing positive in the stimulation of ego. Indeed, no phenomenon in G‑d's world is intrinsically negative, for all derives from Him, and He is the essence of good.
But while we have been empowered to exploit many ostensibly "negative" traits toward a productive and G‑dly end, there also exist forces that are so potent, and whose potential for corruption is so devastating, that we must renounce them as beyond our capacity to deal with.
One such element is pride: we must reject it unequivocally, as any attempt to make positive use of it is bound to fail and be counter productive.
There are times, however, when the positive core of a most negative phenomenon rises to the surface, when its G‑dly essence asserts itself over all iniquitous expressions and corrupting possibilities. Such was the case with the group of individuals who approached Moses in the desert: their "me" instinct asserted itself not in the form of a desire for dominance or corporeal gratification, but in a soul - searing desire to serve their Creator. The cry "Why should we be deprived?!" expressed not a need to have and be, but a yen to give and serve, to recognize and submit to the divine grantor of their freedom.
In their petition, the ferment and "leavening" of their selves was not the antithesis of humble and self-effacing matzoh, but rather its complement. Leaven and matzoh coexisted in their souls, ego giving rise to commitment, self-realization giving rise to an affirmation of man's indebtedness to G‑d.
On the "Second Passover," the festival that came into being out of their "selfish" cry, there is no need to banish leaven from our homes.
For when the self thus asserts itself, it is a welcome participant in our celebration of the freedom we achieved at the Exodus, the freedom to be G‑d's people.