Why would he want to leave me for an old crush?

So my fiance wants to leave me for his old middle school crush that he was so in love with and this was over 10 years ago. She saw that he was getting married and messaged him about getting together after all this time. He told me he was in love with her. I'm like you're leaving me for a middle school crush you don't know much about!!! He was like "it's just something about her". I'm furious to the highest...

Why would he want to leave me for an old crush?
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