What causes a stalker to continuously escalate their behavior to their victim for years?

what is the mental state of a individual who will travel to another country , taking 19 hour flights 3 separate times to try and harass an individual they've only know online for a couple months who rejected them and cut ties with them, then eventually decide to live in their city. And going as far as to pretend to be different people on two occasions to talk to the victim. I genuinely want to know what makes a human being go this far for basically a stranger.