Maybe something like the pressure from their parents or whatever and they decide to enter a fake relationship or where they're feeling they're getting older and their choices growing fewer and thus decides to settle rather than looking for the big true love TM. Or whatever... 

 kwoon:
they decide to enter a fake relationship

Wednesday 3:30 PM - Leads' moms were best friends so they grew up together. The ML takes the FL on fake dates to energize her and make her ex jealous. 

 kwoon:
Or whatever...