Why do people stalk?

2 years ago, I (18M) was a victim of stalking by a girl (17F) on the train, constantly until I took action on it.

It took me a few weeks to realise that it was most definitely stalking.

I would sit on the train and she would always sit next to or near me (even though she had college friends also on the train) and she would also make a strange habit of staring at me and touching me. She was reasonably attractive but it still didn’t take away from the fact that it made me feel uncomfortable.

It wasn’t just on the train either. Somehow she found my TikTok and viewed my profile (she had no way really of knowing my name) and I had random accounts texting me and also my friends and we did some internet forensics and found out the accounts were linked to her.

She started to appear at social events I was also at and places that had no relation to anything she was usually up to.

I ran a test where I took a later train on purpose. I got to the train station at the regular time and missed the first train purposefully. She also deliberately missed the first train.

I also ran another test because I still didn’t believe I was being stalked. I didn’t turn up to the train station on time and took the train 2 trains after my usual. She turned up to the same train as me AGAIN which cemented my belief.

I eventually spoke to her and said she was making me feel uncomfortable. She looked extremely upset and walked off from me on the platform.

Luckily I switched jobs so I didn’t have to take that train anymore. This is when it got A LOT worse. She had somehow found out where I had lived and I started receiving letters, some professing love, some being hateful and, often, both.

I knew her TikTok and instagram so I found out who her father was and told him the situation and it swiftly stopped after that.

It wasn’t too distressing for me but if this happened the other way round or to someone with a different mindset, it would genuinely be traumatising.

Why are people like this? I’m making this post as it’s happening to my female friend currently and she doesn’t want me to tell her specific story. I want advice on how to solve this swiftly for her as it’s putting a lot of stress on her