Because the same way a baby gets worked up over spilling their juice or being taking off gaurd by a loud noise, teenagers will get worked up over childish relationships as they mimic their role models in order to grow up.
Sure it's pointless to an adult. But if I was to mention my problems as a twenty one year old to my mum or my dad's mum. They'd be all "oh lol look at you getting worked up over nothing".
It's because they've gone through it, had closure on it, and have grown past that emotionally.