I think it is all about opportunity and character. Some people will never let themselves get into a situation where there is a possibility of friendship turning EA then PA, whilst others will.
AndyV M38 W36 D7 M 13 years Together 17 years W wants D mid Dec 06 (ring off) W wants separation early Jan 07 (she backdated to Oct 06) EA revealed end March 07 (Nov 06 W first meeting with OM) Hallmark moment "I care for you but am no longer in love with you."