Why did George confide in Slim?

1) Slim was the most respected man on the ranch.

2) Slim presented himself in a way that demanded respect, but it was also obvious that he listened carefully, and most of all..... he understood.

His ear heard more than was said to him, and his slow speech had overtones not of thought, but of understanding beyond thought. His hands, large and lean, were as delicate in their action as those of a temple dancer.

3) Slim gave Lennie a puppy.

In addition....

There was a gravity in his manner and a quiet so profound that all talk stopped when he spoke. His authority was so great that his word was taken on any subject, be it politics of love.