Why can't a sakṛdāgāmin 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 of this matter? Could a Sakṛdāgāmin have such a thought, "I have attained the fruit of a 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," yet in reality there is no such thing as coming and going; it is merely given the name Sakṛdāgāmin.

The meaning of this passage is the same as that of the previous passage on the first fruit. One who has truly attained the second fruit has an even emptier mind regarding the self of the five aggregates, and has no notion of "I have attained the second fruit," let alone clinging to the idea of having attained the second fruit, dwelling on it incessantly, proclaiming it everywhere, fearing that no one knows of it, and if someone does not believe it, becoming so angry as to refuse to speak to them. Such deep clinging is clearly the emotional attachment of an ordinary being, 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, he will return to the human world to continue cultivation. After attaining the fourth fruit of arhatship, upon death he enters 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 return of the second-fruit Sakṛdāgāmin between the heavens and the human world. In essence, is there really any coming and going? There is no coming and going; the matter of coming and going is all illusory and unreal. The five aggregates in the heavens and the five aggregates in the human world are all illusory and unreal, all a false appearance, not the true reality. Therefore, the second-fruit Sakṛdāgāmin also does not truly exist as a Sakṛdāgāmin; the mark of Sakṛdāgāmin is also a non-mark, 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. No matter what attainment we achieve, we should not be moved in mind or give rise to clinging thoughts; otherwise we will fall into the cycle of birth and death, unable to escape.