“And it came to pass when the Lord rejoiced over you to do you good, …so the Lord will delight over you to make you perish” (Deuteronomy 28).
There is joy when a person achieves a goal or sees that he is approaching the goal.
In the world in general, the whole basis was built upon receiving reward in this world and in the next world, and there is no such matter of revoking the doing of Mitzvot [commandments]. Rather, at times a person does more, and at times less. But one does not become a wicked man by subtracting or by adding, since his faith is constant, and on the actions that he cannot do so much, he has excuses why it is justified.
But on the individual level, although he observes, still, because on the individual level, the main thing is the intention, which is in order to bestow without any reward, the body does not agree to this. This causes him alien thoughts, and then he sees that he is wicked, although in terms of actions, he is acting as always. But on the intention, he cannot justify himself, so he must say about himself that he is wicked.
We should ask why this is so. Why did the foreign thought come to him? It is so because the Creator wants the person to walk on the path of truth. And in order not to remain in a state of Lo Lishma [not for Her sake], he is not permitted to observe the Torah and Mitzvot in Lo Lishma and is sent a foreign thought so that if he wants to engage in Torah and Mitzvot, his only way is in Lishma [for Her sake].
It therefore follows that “when the Lord delights to do good,” it means when one engages in Torah and Mitzvot. And “so the Lord will delight over you to make you perish” means He does not let one remain in the state that he is in and observe Torah and Mitzvot like the general public. Why? It is because by this he will have to have the matter of Lishma [for Her sake], for otherwise, the general public will be able to observe Torah and Mitzvot and he will not. It follows that he is even worse than the general public.