The old song is the ongoing song of praise that creation sings to its Lord and Maker. All nature sings that song. The old song is a song that celebrates what God has made. The old song was sung in response to God's acts of creation. The new song is sung in response to God's acts of salvation.
New deliverance brings a new song.
The old song is a song for this age, a song about the present creation. The new song is a song for the coming age, a song about the fullness of the kingdom. It is a song that celebrates because every knee bows before Christ and every tongue confesses He is Lord. It's easy to sing when you have just witnessed a few miracles, and escaped from Pharaoh's army. And sure, it's easy to praise God when you've just found out you're going to be the mother of the Messiah, or when an unexpected refund check comes from the IRS, or when the Publishers' Clearinghouse Prize Giveaway van pulls into your driveway.