Why would a woman cheat with a man's best/good friend?

Cheating is already terribly messed up but why specifically would a woman cheat with the man's best/good friend? Out of other random men, why with the friend?

I'm only asking this because it's now nearly 2 years since reading dad's journal about my parents' infidelity long ago. With my mom's cheating (she was then 20 years old), I wasn't even born yet and when dad cheated back, I was only 2 years old. I know dad cheated in revenge but his journal still didn't explain why mom (his then girlfriend at the time) cheated with his friend. It only said that the reasons were discussed during their reconciliation period and couple therapy sessions long ago. But I don't know why. He knows the reason but didn't put that in his journal.

I still haven't told my parents I found that information. My parents never wanted to share this information with me nor my younger twin sisters. I'm no one to judge my parents (still love them very much) but was wondering if I can one day tell my parents I know their secret or not? I know a cheater does it because they wanted to and are selfish but why specifically with the man's friend out of other men out there?

Why would a woman cheat with a man's best/good friend?
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