Itachi wanted to stay with Sakura, the woman he loved, but to become who he wanted to be, he had to leave.
Sakura, after secretly crying for weeks, came to a final decision.
It was Itachi's dream, his lifetime dream.
What right did she have to take that away from him?
Therefore, she told him that she understood. And she really did.
Even if it hurt like hell.
So they told each other goodbye one last time, and Sakura managed to hide her tears. With one last, loving kiss, they parted the evening before his flight was due to leave.