"Love is not blind. Love is bound and the more it is bound  the less it is blind."

That simply means, you don't idolise your partner when you're in love. Matter of fact, you may know them better than they know themselves. Your relationship transitions from an infatuation stage to a stage of commitment because you know things about them that no one else does.

And for Chesterton, this was a philosophical premise about how we discover our true selves by knowing ourselves through the eyes of others.