Why does Buck choose the civilized world over the world of the wild?

Buck would go back to the wild, except for Thornton. As the book goes on, Buck becomes more and more immersed in the wild with the wolfpack, but he still loves Thornton and wants to be with him also. Buck, however, is moving farther and farther away from civilization.

Buck doesn't necessarily choose civilization. He chooses the love of John Thorton.

Buck's love compounds and becomes more apparent every day. Thornton realizes that Buck will literally do anything he says, even jump off a cliff. Buck gets a chance to prove his love when John Thornton interferes in a bar fight between "Black" Burton and a tenderfoot. Buck nearly kills the instigator, and suddenly he has a reputation in every camp in Alaska. When John falls into the rapids of a river, Buck aids Hans and Pete in rescuing him and breaks three ribs. Later that winter in Dawson, Buck wins a $1600 bet for his master by pulling a sled loaded with a thousand pounds of flour. After that Buck becomes the most famous dog in Alaska. All who see it are astounded, and a man offers to buy him from John Thornton for 1200 dollars, but Thornton doesn't blink before he refuses.