What is the Grinch's heart full of?

The Grinch's heart is initially filled with bitterness and disdain for Christmas and the joy of the Whos in Whoville. However, after experiencing the true spirit of the holiday and witnessing the Whos' celebration despite his attempts to steal Christmas, his heart grows three sizes. This transformation signifies the power of love, generosity, and community, ultimately leading him to embrace the joy of the season.