Lyric discussion by law4 

I think it's about a stalker. The last verse makes that clear ("it's all I think about / I write your name, drive past your house"), and if you read the first part of the song in that light I think maybe the whole, what he talks about like a real relationship, was "only in his head" (as the lyrics say). Some stalkers believe they had a close relationship with their victim, when in fact it was, at most, just politeness or friendliness from the victim's point of view. I think this guy blew an acquaintance or friendship way out of proportion and now is obsessed with the girl and trying to justify his behavior.

@law4 I mostly agree with you here. You know, I was going to say that it seemed like they maybe had a date or two, it didn't go anywhere but the singer thought it did. But as I typed it, I realized, the girl was probably someone he encountered regularly, maybe because she lived close by, maybe because she worked some place he frequented, who knows. As you said, in his mind, a relationship grew when in fact they were acquaintances that knew a little more about each other than most acquaintances would.

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