They are the youngest of the group. Even if they have almost a one and a half year difference, they are still the youngest of the group and therefore can be seen as really close to each other.
. They have similar interests like art, games and photography. Jungkook is really good at art and V really enjoys doing art too. I even saw a video in which he was either asking help from Jungkook or showing him his art.
. V has a very playful personality and just like all the other Bts members he loves Jungkook. He loves to tease him and play with him and they love doing silly things together. In return jungkook also teases him and plays with him too. or just silly things in general. V is also friendly with everyone he comes across and everyone loves him.
. Jungkook has always looked up to his hyungs and still does and it is possible that in the earlier years he felt more comfortable with someone who was the closest to his age, who is V. Like mentioned before, people always feel comfortable around V due to his personality.