Even though Jeon Jungkook was almost completely devoted to dancing, he had always had a love for the guitar. Not the electric guitar, the acoustic one. He had always loved the sound it emitted from its deep insides; so much that he learned how to play it for himself. It was always fun combining his music with that of Jung Eunji's.
Jung Eunji was-and still is-his best friend. Even throughout elementary school, neither of them thought it was weird or unnatural to have a best friend of the opposite gender. They didn't treat each other differently from anyone else-except those they absolutely despised, of course. Jungkook treated Eunji as one of the boys, and Eunji treated Jungkook as one of the girls. It was like this for the first five years of them being together. Then, in middle school, Eunji had taken a particular liking to Jungkook.
At first, it was simple. Her closest female friends giggled with her about how lucky she was and how they wished the most popular boy in school would act with them like he did with her. Then she realized she might like him...at least a little bit. It went down from there. From then, she started clamming up in front of him, her body tensing whenever he came near. And when they got to high school, she avoided him completely. Jungkook didn't understand why until the middle of 9th grade, when one of Eunji's friends got mad at her and revealed all the secrets that Eunji had told her to Jungkook. Including her having a crush on him. Eunji didn't even know about this, and Jungkook still hated that girl to this day. And even though she didn't talk with him anymore, even though they never did anything together, Jeon Jungkook still loved Jung Eunji. Because your heart can never adapt to losing a friend. Even though Jungkook thought she was strictly a friend, he often wondered if she was more than that. And he also wondered if his wonderings would've made her happy if he would've told her. But they were separated now. It was sort of ironic to him how the girl he was with 24/7 could disappear at will. 
When Eunji had gone away, his love for the acoustic guitar had also faded.