Until the end of the time, Diarmuid was never been able to express his feeling to his lord. His farewell is too painful; he can't forget the hatred towards master of Saber. Probably that's the reason why he still has his memory although he was reborn. The pain of the day his lord refused to look at him even for the last time, whenever his chivalry act was doubted.
Probably it was the best punishment for him.
He told Saber that he doesn't want to grow up because that means that his time will be over when he doesn't realize it. Saber just agree because she said that living like a kid is way more fun—although Diarmuid bet that his best friend will never understand that. He glanced over someone who just passed behind him, a student just one year above him.
The only one who has the memory is him, that's why Kayneth didn't recognize Diarmuid when both of them met in the schoolyard. But knowing that his lord is here is enough for him.
He tried to get Kayneth's attention despite his senior never really care for him, because he just hanged out with his friends. Tokiomi Tohsaka and Kariya Matou, Diarmuid never thought that all of them will be here. He can't find some of them, such as Lancelot, Gilles de Rais, and master of Caster. It's a good thing that master of Saber isn't here; it will be too much for him. He can't find Sola-Ui too, which he considered a bonus because it'll be troublesome if history repeats.
Someone might mistook him as stalker because occasionally he unconsciously follows Kayneth everywhere and watch him for afar, smiled whenever Kayneth smiles and enjoying his present life. He's grateful for this, but actually Diarmuid wants more than this. He doesn't want to be a mere watcher; he wants to be the source of Kayneth's smile.
Will it continues like this forever? When God gave him chance to tell his feeling after it was over? His hope of meeting his lord once again was granted and the only thing he can do is watch his lord. Sometimes he thinks that this is the best since he is cursed with E-rank luck and probably he'll make his lord sad, but his ego (which sometimes he mistook as chivalrous spirit) never let him to stand still.
Back then when he was still a servant, he never really wants=ed to confess to his lord because there is no place for him inside his master heart. As a knight, he never despises the fact that his lord is all over his fiancée, made him admire his master for his undying love.
Probably he should just protect his master.
But he failed terribly, he made the worst choice back then. He failed as a knight and servant. That's why he wants to correct his mistakes back then.
The biggest regret is how he never be able to deliver his feeling to his lord, who will never noticed. Even until he met him again, his lord will never notice it because he's just a coward who doesn't want to make any more mistakes if it's related to Kayneth. Diarmuid is afraid if the smile disappears from Kayneth's face, something that he can't do.
He was left in despair, even after a year has passed and Kayneth is now a third year who's going to graduate soon. That's why he doesn't want to grow up because he'll never be able to catch Kayneth. Will it ends the same way as before?
No.
This is going to be different.
No one can replace the special place Diarmuid held his feeling for his lord. When now he can stand in the same plane with his lord, he has the chance. Probably it's only less than one percent, but it's better than nothing.
If he never changes from a coward, his chance is perfect zero.
When Diarmuid saw Kayneth walked with Tokiomi and Kariya, Diarmuid yelled his senior name and grabbed Kayneth's slim wrist. It didn't change at all, his lord was always thin and fragile, made him want to protect him at all cost, especially when nothing is between them.
Like right now. He confessed his feelings, throwing all his fear away. He could see Tokiomi and Kariya went speechless and told Kayneth that they will leave Kayneth and Diarmuid alone. After saying that words, suddenly fear covered him and he's too afraid to hear the reply. It doesn't need to be positive reaction, but Diarmuid doesn't want if Kayneth rejected his existence.
"Finally you said it, Lancer. How long are you going to make me wait?"
But the chance is more than zero.