WHY did he do it?

I think Kishimoto wanted Itachi to be a villain at the start but then he saw his popularity

I still can't understand itachi

this was when my boy made up his mind that kakashi wasn’t enough for his mission

To show him how week he was

Brainwashing at its finest

People saying Hatred is need for him to awaken power or strong is WRONG we already learn this is NOT how sharingan and mangekyou awaken to Uchiha. What itachi feeding in his head is Not making Sasuke strong but leading him to down path or worse.

To make him strong because ultimately it would be up to him to kill Madara/ Obito

why jiraya just watch😭 i forgot

Because Sasuke was weak and lacked hatred. Itachi probably thought Sasuke would get strong enough to at least have 3 tomoe by the time they met. Seeing that he was a genin with 2 tomoe, he probably decided to accelerate Sasuke's growth. Which he successfully did.