I heard from a Rav in a shiur that he saw written somewhere in the name of the Ba'al HaTanya an explanation for the song Echad Mi Yodeya at the end of the Haggadah. It seems a little incongruous to start counting seemingly random things to do with Judaism.
He explained it with a parable that when someone wants to show off how proud they are of their ancestry/heritage, they start showing various emblems and artifacts from their past. They show off this medallion, this ancient manuscript, these articles of clothing. That's what we're doing at the end of the Haggadah. After solidifying our relationship with Hashem, we contemplate how proud we are of our heritage. We start listing off things that make us special 1. One G-d in Heaven. Two stone tablets. Three forefathers...etc.
Anyone know the source for this explanation? On a side note, in my attempts at finding this, I found that Lubavitch chassidim don't have the custom to say this song. I find it interesting then that there would be an explanation of it from their founder.
1 The Rav admitted that 9 months of pregnancy and 11 stars in Yosef's dream need clarification why we're listing them.