What's with the 'you're my kryptonite' line?

I was in a relationship with this man, but twice now he has suddenly stopped talking to me. Both times he tells me it's because he's afraid of how I make him feel. He says I'm his kryptonite and that he doesn't want to feel weak or vulnerable. Is this just a case of him not being that into me? Or does he have issues? Do some men really freak out when it comes to truly caring for someone?
What's with the 'you're my kryptonite' line?
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