Even though Lady Macbeth is expected to be happy, she is not. She felt alienated from her husband even though they generally have a good marriage. Macbeth knows that he is guilty and even when he is reaching success, he just knows that he cannot be fully happy because of what he has done.
He has killed Duncan and he cannot help but feel remorseful about what he has done. What he started feeling was since he felt that he has already lost his soul when he killed Duncan. There is no reason why he cannot keep on keeling in order to feel that the loss of his soul is worth it.
