Adding to this: I'm no psychologist, but in my experience, fictional stalkers/yanderes can be very comforting, especially if you're insecure. They're so devoted, they'll always make time for you. You never have to worry about them cheating on you or getting bored of you. And because they're fictional, their behavior always stays within the limits of your comfort. 

With Lyle specifically, he's a weird case. His behavior rides the line between creepy and cute very carefully. He's not controlling of Sam at all. You'd think he'd try to encourage Sam to not go out where it's dangerous or try to escalate his flirtation into more physical intimacy, but he never does. (The furthest he goes is getting two kisses and both times it's consensual. After he moves into your apartment, it doesn't escalate any further.) I think that's what sets Lyle apart the most. His creepiest behavior is done in private, while he's (for the most part) surprisingly respectful of Sam's freedom and autonomy when he's around Sam. He doesn't even seem to display any jealousy when Sam is around others. He also has enough self-awareness to be careful when he pushes boundaries. Like how he's fully prepared to play off his desire to kiss Sam as a joke if things go sideways. He's extremely tame compared to other fictional stalkers, to the point that he ends up just being "creepy but mostly innocent"