He told me I'm his kryptonite?

I know the phrase "you're my kryptonite" is a metaphor for being ones weakness, but what are the reasons a man would call a woman his kryptonite? A close friend of mine just told me that I'm his kryptonite but I don't get why. We flirt and talk on a regular basis, but not really on an emotional level. I know he likes me, but I didn't think I had a weakening effect on him because we haven't really gotten close and talked about personal stuff very much. How could I possibly be his "kryptonite" ?
He told me I'm his kryptonite?
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