וַיִּשְׂמְח֤וּ הָעָם֙ עַל־הִֽתְנַדְּבָ֔ם כִּ֚י בְּלֵ֣ב שָׁלֵ֔ם הִֽתְנַדְּב֖וּ לַיהוָ֑ה וְגַם֙ דָּוִ֣יד הַמֶּ֔לֶךְ שָׂמַ֖ח שִׂמְחָ֥ה גְדוֹלָֽה׃ (פ)
Then the people rejoiced, for that they offered willingly, because with a whole heart they offered willingly to the LORD; and David the king also rejoiced with great joy.
Read kabbalistic (mystical) interpretation of I Chronicles 29:9: and classical mystical sources.