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.
Another reaction could be that Sakura tries to follow Sasuke. She believes that she can change his mind or that she can be of help to him. She might leave her own responsibilities behind for a while to chase after him, thinking that their bond is strong enough to overcome whatever is making Sasuke leave.
In some fanfictions, Sakura may be determined to find Sasuke and bring him back. She could set out on her own journey, facing various challenges along the way, and growing both as a person and as a ninja in the process of trying to reunite with him.
She may initially be angry. Sakura is a proud and strong character. She could confront Sasuke and Ino, demanding an explanation. After that, she might decide to become even stronger, both in her ninja skills and her emotional resilience, to show Sasuke what he's missing.
Sakura might be heartbroken. She's been in love with Sasuke for so long. She could initially go into denial, thinking it's just a temporary thing. Then she might get angry and vow to prove herself to Sasuke, perhaps by becoming an even stronger ninja.
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.
One possible plot could be that Sasuke leaves Sakura to pursue a solo mission of redemption. He feels that his past actions have left a stain on his honor and he must atone for them alone, believing that being with Sakura would only hold him back from his self - imposed penance.
One common theme is unrequited love. Often, Sakura has deep feelings for Sasuke which are shown through the symbol of sakura leaves. Another theme is redemption. The sakura leaves can represent new beginnings and Sasuke's journey towards redemption. For example, Sakura might use the beauty of the sakura leaves to remind Sasuke of the good in his life.
One possible reason could be Sasuke's pursuit of power. In many fanfictions, his thirst for greater strength might lead him to leave Sakura behind as he embarks on a solo journey to train in more dangerous and secluded places. He may feel that his goals are too dangerous for her to be involved with.
There are quite a few fanfics out there. One I remember has Sakura's growth as a ninja intertwined with her feelings for Sasuke. It shows how her experiences, like dealing with the fall of Sakura leaves in the village, symbolize her changing relationship with Sasuke. Another one focuses on Sasuke's return to the village and how Sakura, with the reminder of the sakura leaves (a symbol of new beginnings in their village), tries to reconnect with him on a deeper level.