Do you mean why did Macbeth forget to *leave* the daggers? (If so, that's an excellent question). Macbeth does not tell us directly, but we know he has been extremely ambivalent about killing Duncan. We also know that he is already exhibiting signs of guilt (he thinks he hears a voice saying Macbeth does murder sleep), and that he is not thinking clearly. He has fought numerous battles, so he should not be afraid of blood; neither should his mind wander after a killing, yet it does. He is not one to make mistakes in battle, so this mistake is all the more poignant. Perhaps the daggers return with him because he cannot leave behind him what he has done.