My 18-year-old daughter’s first boyfriend (when she was 17) was like that. He would have meltdowns if she didn’t answer his messages while she was in class. He told her she was the only reason he would keep living. We helped her get away from him (fortunately, he was in another state - one of those Internet relationships), and he’s still alive, as far as we know.
Now, she promptly dumps any young man who shows signs of emotional neediness. “He needs to get a pet!”
A good reason relationships in 1966 were different than 2016. In my teen love years me 17, her 14 I was in the Army. First year at bases all over the US and only afford a monthly phone call, but daily letters. Half of second year I was overseas with letters only. So by the time we were 'a courtin' 18 months after meeting we had only been person to person a few times when I could get on leave. No chance to be hooked at the hip with a smartphone.
Yeah, that’s a highly manipulative, controlling feature, that whole “I will die if you don’t make me your top priority” thing. Massive red flag.