Sotah 35a: David was punished because he called words of Torah "Zemiros." Hashem rebuked him. It says about Divrei Torah "wink your eye (for one moment, neglect) them, and they are gone." Do you call them songs?! I will make you stumble in something that even children learning (Chumash) from their Rebbi know! "He did not give (wagons) to (the Leviyim,) Bnei Kehas, because they carry the most Kodesh things (rather, they will carry on their shoulders)." David brought the Aron on a wagon.

Radak: Where I was exiled and wandered, in place of the sadness I had from Resha'im, when I remembered Your Chukim, they were songs for me, and I delighted in them.

Malbim: Even though I was a stranger chased from my place, still, I learned Your Chukim with songs and Simchah. Even Chukim, whose reason is not known, I learned b'Simchah.