Albert Barnes Commentary 1 Chronicles 29:9

"Then the people rejoiced, for that they offered willingly, because with a perfect heart they offered willingly to Jehovah: and David the king also rejoiced with great joy." — 1 Chronicles 29:9 (ASV)

The people rejoiced because they offered willingly: that is, the munificence of the princes and officers (1 Chronicles 29:6) caused general joy among the people.