Why do women cheat on their best friend?

On several occasions I've heard from those whose relationship had ended, the original reason was that their other half was cheating on them with her best friend. Can you explain to me why as a man you would do this, cheating on your partner with her best friend and as a woman too, cheating on your best friend by seeing her partner. This is obviously so wrong, why do it?
Why do women cheat on their best friend?
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