The moral theories of Hutcheson, David Hume and Adam Smith, identify sympathy or love, perhaps combined with other psychological factors (such as disinterestedness, calm judgment, or impartial spectatorship) as the psychological foundation of all morality. Kant always had much respect for these theories. But it was a crucial turning point in Kant's thinking about morality when he decided that no such theory could give an adequate account of morality.

Kantian Ethics (2008) - Ch. 2. Moral Worth