Sometimes, a person will flit from one relationship to the next without any breaks in between. 

If she's the type of person who does this, she might not want people

to start suspecting that she doesn't know how to find happiness with herself (or in a relationship). 

Worse yet, she could be branded as a slut. This could lead to lies

about why her last relationship fizzled out. 

She left him for a new guy, but she starts thinking about her ex more and more. 

The new guy isn't working out, but does she really want to admit that

she's regretting her decision? 

This could be a matter of pride or it could just be a matter of not knowing how good she had it before. 

Either way, it would suck if

other people found out she felt like she made a mistake. 

Slut shaming is a thing, and if a woman leaves a man for another one,  she could be viewed as a whore or worse. 

No matter the reason for leaving, this

could mean that she chooses to keep her reasons for moving on a secret—even if they're VERY good reasons. 

If she was with him for a long time, friends and family might question WHY she stayed in the first place. 

If the guy was a waste of time, she

might not want anyone but her knowing that, even if moving on quickly with a new guy doesn't look great.

In the end, she might choose to keep her reasons

for breaking up with him a secret, AND keep her new a secret too.