If Sakura were to give Naruto a token made of sakura leaves as a symbol of their friendship, and then later break off all contact with him and leave the village, Naruto would be heartbroken every time he saw a sakura leaf. The once - positive association would become a painful reminder.
One possible fanfiction could be that Sakura, in a moment of great sacrifice, uses her chakra to turn into sakura leaves that Naruto has always associated with her beauty and kindness. But this transformation is a final goodbye as she is sacrificing herself to save the village. Naruto is heartbroken as he watches the sakura leaves scatter, feeling like he has lost a part of himself.
The relationships within the village would be severely affected. His friends would be heartbroken and split into groups, some trying to find him, others blaming themselves. The new generation of ninjas would also lack a role model. And the balance of power in the ninja world could shift, as other villages might see Konoha as weakened without Naruto and try to gain an upper hand.
He could leave because of a broken heart over a lost love. If Naruto had strong feelings for someone in Konoha, like Sakura, and she chose someone else or rejected him in a cruel way, it might be too much for him to bear. So he decides to leave the place full of memories related to that person.
She might be heartbroken. Sakura could initially be in shock and then fall into a deep sadness. She may blame herself for Naruto's leaving, thinking that she could have done something differently.
In some fanfictions, Naruto might initially withdraw into himself. He could focus more on his training to distract from the pain. For example, he might go on long solo missions or intense training sessions in the woods, away from the reminders of Sakura.
Naruto might initially be in denial. He's so used to having Sakura around and believing that they have a future together. So when she leaves for Sasuke, he might think it's just a temporary thing or that she's been influenced by something or someone. But as time passes and he realizes it's real, he would be deeply hurt. He might also start to question his own self - worth, thinking that he wasn't good enough for Sakura. And he might even try to win her back at first, but then slowly come to accept the situation and focus on his other responsibilities like being a ninja and protecting the village.
The village would likely be in chaos. Naruto has been a key figure in protecting the Hidden Leaf. Without him, there would be a power vacuum, and enemies might see it as an opportunity to attack. The villagers who once relied on Naruto would be filled with fear and uncertainty.
She could be heartbroken. Sakura has always loved Sasuke deeply, so his leaving for Naruto would be a huge blow to her. She might initially be in denial, not believing that Sasuke would choose Naruto over her.
Maybe Naruto discovers that the village he has been trying so hard to protect has been using him all along. He might find out some dark secrets about the Hidden Leaf, like they were involved in something immoral and were just using his power, which would shatter his idealistic view of the village and leave him heartbroken.