I've been asked the same question once by a girl. Below was my reply. Pretty good results only because it was consistent with my routine and that I'd broken her shield at the same time.
HB10: "Why do you like me?"
Slique: "Why do you wanna know?"
HB10: "I don't see any reason why i'm so attractive"
Slique: "Do you believe in magic?"
HB10: "Huh? What's that got to do with.."
Slique "DO you believe in magic?"
HB10: "Yeah I guess so"
Slique: "Then there shouldn't have to be a reason then. Science is about reasoning. Magic is about wonder and intrigue. [Insert Cold reading here] You've grown skeptical over the years, and by all means you have every right to be. [Lean forward and talk slower... deeper] But for once in your life, there comes a person who's made you feel good and comfortable that for one brief instant [Pause a beat] you've forgotten all your worries in the world. [Dramatic pause]. That's magic." etc. etc.
Note: if she says "No" u can reply with the "Old Shaman's tale":
Slique: "The Greeks believed when lightning struck, Zeus was angry at them. Often these inexplicable things were attributed to magic, because they could not find a reason for them...." [then go into the above routine]
Moral of the story: don't give her a concise reason, use push and pull, beat around the bush, keep it fun, keep it funny, and above all keep it interesting.