Sometimes the actors know their characters a lot better than the writers. For example, in the empire strikes back, the original script had Han saying something else, whereas Ford came up with the ‘I know’, which fits much better with his character.
Kershner: (Tries it out) “I love you.” And you say, “Just remember that, Leia, because I’ll be back.” You’ve got to say, “I’ll be back.” You must. It’s almost contractual!
Ford: If she says “I love you,” and I say “I know,” that’s beautiful and acceptable and funny.