May I be so bold as to suggest that’s it’s because young people feel they need to get “the perfect” shidduch and that doesn’t materialize because as humans everyone has their faults. And when you spend more time together the negative characteristics come up. What is a girl or boy supposed to look away from when they see their chosson/kallah has some anger management issues or not enough sensitivity or is conceited or any other of the myriad negative but human characteristics?
What could a person look away from?
Well, the easiest thing to do is call the whole engagement off.