Firstly I'd like to state that there's no justification for what this guy is doing. Making somebody's life intentionally and persistently miserable can have no justification.
The story basically starts with a boy falling in love with a girl that won't love him back. She's still using interaction with him for her purposes though. Then the boy grows into a man and his addiction to a person gets deformed into addiction to harming this person in non-violent ways. That's what happens to some people. Happens in marriages too. If teenagers are weird adults are way weirder.
I went similar route. I had it easier than this guy. I fell in love at the age of 16 (so later than him). I already had one episode of unrequited love earlier (probably around age of 12) which was extremely wisely ended by my love interest and friend by cutting all (and I mean all) contact with me when she learned about it.
This second girl I spent about 7 years addicted to. I was her friend. Even after she learned that I'm in love with her she still kept me around. Used me for comfort. Used me even as a rebound when she needed to ditch her second boyfriend whom she loved but who apparently got bored with her (in my state of mind I was more than happy to help).
Who knew how it all would end if I haven't made conscious decision to cut all contact. It was really hard. It took a year of manually steering away my thoughts from her. I had to vilify in my mind the person I loved. I had to ignore all pings from her (fortunately it was before Facebook, so just emails and texts) which fortunately wasn't that many of. When you are an addict every dose is something you need to be very careful about. She had new boyfriend at the time so she eventually forgot me and my brain formed mental scar tissue around pathways carved over the previous 7 years. I got comfortable, even happy. I met a girl who loved me and moved in with her (fortunately didn't get addicted again).
10 years later I meet object of my addiction on the street 100 meters from my new home where I live now. She's with another man, she's pregnant. She just accidentally lives in my neighborhood. I'm scared shitless. Fortunately I haven't met her again. I have no idea if she moved or still lives around. I don't get out much. She made one more attempt to contact me (through Facebook because I accepted her friend request, because everyone has capacity to be stupid occasionally). I answered with "?" She responded with something with something about staying at home with child broadening horizons. I responded with image that basically implies that she finds her child boring. Which she somewhat acknowledged and fortunately haven't contacted me ever since.
I can imagine many points in my history when I might have turned into a stalker. If I was more lonely, had less support from my family, was less intelligent, was less introspective, had less knowledge or interest in psychology, had less awareness about how my feelings are not necessarily my identity... It could have turned ugly for everybody.
Brother of my mother was an alcoholic. I might be prone to addiction. I binge watch series. I binge game. I'm addicted to news. But I'm also addicted to learning new things and to figuring why stuff doesn't work when it should. The only addiction that severely harmed me was to a person. But I see I lucked out when I read stories like these.
Issue of stalking is real and I hope in the future there'll be a lot more research about this and that efficient ways to mitigate this will be developed. I think whatever the solution is it involves stalker getting psychological or psychiatric help.