Most Helpful Guy
Mental illness, loneliness. I know a girl who has like several mental illnesses even in relationships with other people all she talked about was her ex, she cheated on every guy she dated with him and always would run back to him.
For being lonely, when someone doesn't have attention from the opposite gender as a whole, constant action from one or a few will be seen as romantic action. This is common in men I noticed1
Most Helpful Girl
It's kinda like an addiction. First it starts off as a present presence in your life. Then you start to think that your happiness is dependent on that other person. Which can slowly/quickly turn into not being able to function or live with out that person.
I would imagine that people who develop these feeling have a mental disorder of some degree1