"Best Friend's Brother" is an electropop song that contains influences of teen pop and dance-pop. The song runs through a dance-oriented beat and features a rougher sound than the previous single, "Beggin' on Your Knees". The theme of this song is center around exploring emotions and managing friendships. The lyrics are about a girl who has a crush on her best friend's older brother, and even though the girl wants to pursue a relationship with him, she doesn't want to jeopardize her friendship with her best friend. While being interviewed by Neha Gandhi for Seventeen, Justice discussed the background story for the song:
