Assuming you are referring to the Korean boy band BTS, Jungkook may have felt lonely when Jin left because they are close friends and bandmates who have been working together for a long time. As a group, BTS has often talked about the strong bond they share and the importance of their friendship.
When a member of the group is absent, it can disrupt the dynamic of the group and affect the relationships between members. In addition, Jin is the oldest member of the group and has often taken on a leadership role, so his absence may have been particularly noticeable.
Furthermore, Jungkook and Jin have shared many experiences together as part of BTS, such as performing on stage, recording music, and traveling around the world. When one member of the group is absent, it can be difficult to replicate those experiences with someone else, which may contribute to feelings of loneliness.
Overall, Jungkook's feelings of loneliness when Jin left may have been a result of the strong bonds and shared experiences that they have as members of BTS.