Why can a Sakṛdāgāmin not have the thought “I have attained the fruit of Sakṛdāgāmin”?

Original text of the Diamond Sutra: 须菩提。于意云何。斯陀含能作是念。我得斯陀含果不。须菩提言。不也。世尊。何以故。斯陀含名一往来。而实无往来。是名斯陀含。

A literal translation: The Buddha asked Subhūti: What do you think about this? Could a sakṛdāgāmin have such a thought, "I have attained the fruit of sakṛdāgāmin"? Subhūti replied: A sakṛdāgāmin cannot have such a thought, World-Honored One. Why is this so? The sakṛdāgāmin is called "one who returns once more," yet in reality there is no such thing as coming and going; it is merely given the name of sakṛdāgāmin.

The meaning of this passage is the same as the preceding passage on the first fruit. One who has truly attained the second fruit has an even emptier view of the self within the five aggregates, and has no notion of "I have attained the second fruit," much less would such a person cling to the idea of having attained the second fruit, dwell on it constantly, and proclaim it everywhere for fear that others might not know; and if others did not believe it, such a person would become angry and refuse to speak to them. Such deep attachment is clearly the emotional clinging of an ordinary person, and has nothing to do with the first or second fruit.

One who has attained the second fruit will, after death in this life, be reborn in the desire-realm heavens to continue cultivation; after death in the desire-realm heavens, that person will return to the human world to continue cultivation; and upon attaining the fourth fruit of arhatship, at death that person will enter nirvāṇa, no longer taking rebirth in the three realms to undergo suffering, having been liberated from the suffering of birth and death in the three realms. This is the single coming-and-going between the heavens and the human world of the second-fruit sakṛdāgāmin. In essence, is there really any coming and going? There is no coming and going; the affairs of coming and going are all illusory and unreal. The five aggregates in the heavens and the five aggregates in the human world are all illusory and unreal, all false appearances, not the true reality. Therefore, the second-fruit sakṛdāgāmin is not truly a sakṛdāgāmin either; the mark of sakṛdāgāmin is also a non-mark, and it is merely provisionally named sakṛdāgāmin. We need not cling to a false name and mark, wasting our minds in vain; we should extinguish the mark of self and the mark of sakṛdāgāmin, and remain unmoved in mind and free of clinging thoughts no matter what attainments we achieve; otherwise we will fall into the cycle of birth and death and be unable to escape it.