Why does Gray Beaver trade White Fang for the white man's whiskey?

Beauty comes to Gray Beaver's camp to try and buy White Fang. Gray Beaver refuses to sell. Beauty gives up, but returns often, and always with whiskey. Soon, Gray Beaver is addicted to the whiskey, and slowly loses all of his money to his thirst. Again, Beauty tries to buy White Fang, and this time he is paying in bottles of whiskey. Gray Beaver agrees to the trade.