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Because then she has to gauge whether she's into you enough to think about breaking up with her bf, which could be emotionally loaded, which would mean that deciding how she really feels is more effort than she wants to expend. And if she decides to stick with the bf, she might feel better deciding that it's not so much that she's into you, as you're into her, which makes her curious/horny, which she doesn't want to admit, so then it's your fault that's she's confused.

Sound completely irrational and convoluted? That's because I had a friend in high school who was in a similar position, and this line of reasoning was essentially what the girl screamed at him when she melted down.

__________________
Dull sublunary lovers love,
Whose soul is sense, cannot admit
Absence, because it doth remove
That thing which elemented it.

(From "A Valediction: Forbidding Mourning" by John Donne)