Why did the people rejoice as gifts were given for the temple in 1 Chronicles 29?

Answer: Their leaders gave so willingly

The people rejoiced at their leaders' willing response, for they had given to the LORD freely and wholeheartedly; David also rejoiced greatly.

“And the people rejoiced at the willing response of their leaders, for they had given to the LORD freely and wholeheartedly. And King David also rejoiced greatly.”— 1 Chronicles 29:9 (BSB)