Look at full-on stalkers. They obsess over someone - maybe it's a celebrity, maybe it's their ex or a colleague, maybe it's just some random - for years. You know, you listen to the way the victims talk about them, and there *is* a relationship there. The victims are aware of this person who's always staring in the windows from across the street and occasionally breaks in or takes laundry from the line or whatever, they're very acutely aware of it, of how this person works, of what this person thinks of them.
But the stalker isn't aware of any of that, at least initially. It's all anger and passion and it's directed not at the individual being stalked - even though they're receiving the attention - but at this ideal version of them that the stalker probably knows deep down doesn't exist. It's painful to let go of our obsessions; for some people it becomes stalking, for some it becomes hoarding, for some it becomes a hobby or a set pattern of doing things that we get really upset about breaking, but we all have obsessions, and we can all fail to see them for what they are. It's like tunnel vision: you're so focused on the goal you fail to realize that you don't really know how to achieve it or what, if anything, it really would be. You're just "getting through this".
When the goal is getting with someone it's painful to think that, because of a bad day or just bad timing, the little flutter you get whenever you see them is never going to be anything else. Normally we get over that; but you never quite forget how they made you feel, and the danger is that you mistake this continuance of feeling for something they must also feel. You become convinced that you're in love and it's not going to go away, but the other person hasn't really given it a second thought.
And then one night you're masturbating on their children's beds while they're on vacation and it occurs to you that you got arrested for this before and maybe they don't love you.