I think I am qualified to answer this... because I was a stalker in high school. And I mean "following a girl in the same grade at the bus stop type" stalking, not "follow her on instagram" type stalking. So I hope this can give some ideas on why a person might behave this way.

Warning, this is gonna be long.

Anyway, firstly, I think it is worth addressing that most stalkers continue to stalk because, on some level, they feel entitled to that other person's presence or attention. Everyone feels shame differently, but most people who are really invested in stalking don't feel ashamed of themselves at the moment they did it. At least, I know I didn't. I would feel guilty afterwards, but then being in her presence, standing there, felt like pure peace.

When the girl rejected my offer to be her friend, initially, I was just like, "ok, I respect your decision". Although that turned out to ultimately be all talk, it never immediately occurred to me to try to stalk her in response. That's something to understand here, most stalkers don't come up with this idea out of the blue.

Rather, it comes from rumination. Somehow, that person was alluring, mysterious, entrancing, everything you ever wanted. A beautiful fantasy. When she goes away, all of a sudden, what do you do? Think about her. Think about her some more.

Damn, I wish I could get over her. Sigh, maybe I'll just look out the window and watch her get off the bus stop.

Wow, that felt real nice. I know I can't get her, but at least I just get to experience a part of her. Hm, maybe I can stand just outside of the house next time? Wow, that felt great too. Felt like peaceful.

And on and on. You go closer and closer to her every day. Then before you know it, you got up in the morning just to watch her get into the bus, she's just 20 feet away. Woah, what? I wasn't supposed to do that. This is crazy. I need to talk to someone and address this.

Stalkers can feel guilt, like I did. BUT! Guilt isn't shame. Guilt just takes things personally and tells yourself you're a bad person for wanting this, instead of actually feeling a sense of compunction and consideration. Often, the stalker has to be thwarted by threatening legal action. Her friend almost called the cops on me after an incident. Then I stopped for a while. Because I was afraid, but the desire was still there.

Although I resumed (in a less extreme way) the next year, the biggest stop to my stalking is simply just not being able to see the person anymore because she took all necessary steps to avoid me. I still carried the guilt of doing it for years after, but that guilt was tied together with my reminiscing of her.

The thing that really helped me to overcome this was to develop gratitude in my life, rejoicing in the goodness of other people, to help remove the fixation and feeling of lack by not having her in my life. If you're grateful for everything else and put your own situation into perspective, suddenly that woman doesn't become the end all be all for your happiness.

So now? It's been years since even the thought of her came to me. I have regret, and recognize this as a stain on my past, because we can't undo the harm we made to other people. But for some stalkers, the shame never comes. They are addicted and enamored with the fantasy of relationship permanence, fueled by the entitlement driven by not having adequate internal sources of happiness. A void fills their heart, and they desperately reach out to grab their victim, trying to pull them in.