Justin lived close to the beadles and then started to date catilin beadles and that's how he met Christian by always being with caitilin. Then when Justin got discovered and his songs cane out and he became popular he started to go on tour a lot so I guess he thought it was best that him and catilin just be friends since he would really never be home. Even though catilin and Justin don't date anymore and they are still very close and Christian is still very close with Justin too.
