To contract ties of Friendship with any one, is to contract Friendship with his virtue. There ought not to be any other motive in Friendship.
 Henry David Thoreau, A Week on the Concord and Merrimack Rivers (1849). copy citation

Context

“They do not charm us with their eyes, But they transfix with their advice; No partial sympathy they feel, With private woe or private weal, But with the universe joy and sigh, Whose knowledge is their sympathy.
Confucius said: To contract ties of Friendship with any one, is to contract Friendship with his virtue. There ought not to be any other motive in Friendship. I have a Friend who wishes me to see that to be right which I know to be wrong. But if Friendship is to rob me of my eyes, if it is to darken the day, I will have none of it. It should be expansive and inconceivably liberalizing in its effects.” source