The slumbers of Nature are over: “Lo, the winter is past, the rain is over and gone, the flowers appear on the earth, the sound of the turtle-dove is heard…” (Shir HaShirim 2:11-12).
The spring-like atmosphere calls for a reading which envisions the same spirit of renewal, hence the choice of Shir HaShirim.
But this is not all. Pesach is the end of the darkness of the Egyptian bondage, the emergence of the people of Israel into the springtime of national renewal and hope.
This is the mood of Pesach – not just the Pesach of the past, Pesach Mitzrayim, but the Pesach of the future, Pesach L’Atid, when the people of Israel and the entire world will emerge into the messianic redemption in which all the Universe will see the flowers appear and the birds sing.