The Grinch in Dr. Seuss' book "How the Grinch Stole Christmas!" is mean due to his hatred for Christmas and the Whos, as well as the fact that his heart is "two sizes too small." Other reasons include his scorn for happiness and his ill temper.
The Grinch is portrayed as a recluse who looks down on the Whos from his cliff with bitterness as they enjoy their Christmas festivities. His mean behavior may also be due to his seclusion and lack of participation in the Christmas celebrations with the Whos. The Grinch hates all of the activities that the Whos participate in, including the Christmas gift exchange and the preparation of their annual feast.