Why is it not a good idea to say "I feel like I've known you my whole life", and why do many people say it to people?

Dating experts suggest not to say that to your date. Why do they suggest against it? Why do people often want to say that to their date in the first place?

Why is it not a good idea to say "I feel like I've known you my whole life", and why do many people say it to people?
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