The specific reason that requires this specific person
The fake dating setup is only as strong as its logic: why does this protagonist need to pretend to date, why can that pretense only be performed by this specific person, and why is the arrangement mutually beneficial rather than one-sided exploitation. Writing a strong setup requires answering all three questions with specific rather than generic answers. The setup where one person needs a date to a wedding and any presentable person would do is weak; the setup where one person needs to convince their ex they have moved on and specifically needs someone who already knows the ex, or fits a type the ex would be jealous of, or has a specific connection to the social world in question — this is stronger because it explains why this specific fake relationship is necessary rather than merely convenient.