On matzot and bitter herbs they shall eat it. (Bamidbar 9:11)
Both of these verses discuss the Pesach sacrifice, and both mention bread. The second verse requires us to eat the Pesach with matzah. The first verse requires separation between the Pesach and chametz. Why is the connection between bread and the Pesach so important that two separate mitzvot are needed to define it? To answer this, let us look at another sacrifice - the "todah" or thanksgiving offering - which also has a strong connection to bread.
There are extensive parallels between the thanksgiving and Pesach offerings. Both are:
- "shelamim" - sacrifices which are eaten by their owners anywhere within the walls of Jerusalem.
- distinct from normal "shelamim" in that you must finish eating them by midnight after the day you have sacrificed them (for normal "shelamim" you are allowed an additional 18 hours).
- closely involved with bread: Pesach as we mentioned, and the thanksgiving offering in that it must be sacrificed along with 36 loaves of bread which then are eaten along with the sacrificed animal.
Beyond the practical parallels, there are thematic parallels between the sacrifices. You sacrifice the thanksgiving offering to thank God for saving you from one of four dangerous events (Tehilim 107, SA OH 219): being imprisoned, lost in the desert, dangerously sick, or traveling at sea ("yordei hayam").Some or all of these events apply equally to our experience leaving Egypt. Egyptian slavery was certainly a form of imprisonment; upon leaving we endured a long and difficult desert journey; the 10 plagues are called a "disease" (Shemot 15:26); and traveling through the split Sea would likely qualify as "yeridah layam". Thus, the background to offering the Pesach and thanksgiving sacrifices is very similar.
Why all these parallels?
It seems that the Pesach offering is simply a thanksgiving offering corresponding to a national/historical rather than a personal salvation. As Sforno explains (Vayikra 7:13), the purpose of the many loaves of bread you eat with the thanksgiving offering is to require you to have a communal meal to publicize what God has done for you, since no person can eat 36 loaves in an evening singlehandedly. The Pesach must similarly be eaten at a meal commemorating what God has done for us - the Seder.
In summary: Pesach and thanksgiving offerings are linked by their relation to bread, their sacrificial procedure, the events triggering them, and the manner in which they are eaten. It seems they are the exact same offering, only offered in response to different events - personal, or else communal.
UPDATE: A friend of mine (D.Z.), who I assume does not read this blog, sent me a dvar torah in which he too made the connection between yetziat mitzraim and the 4 reasons for korban todah. I asked him where he'd got the idea. He said he'd thought of it himself, but afterwards found the same idea in the Maharsha, Brachot 54a.