Because she's Olivia and sometimes she acts like a damn robot. She doesn't like to open up even though she has people who love her. Also she needs to see a therapist or something. She's been through too much and she just holds it in and keeps going until she explodes and kills somebody.

Thought this storyline was abandoned, it obviously has to have some connection to her "psyche" yet , nada. I'm hoping it will come back around.
It's an important example of a woman's choice and also of why they make the choice. Maybe in the end.......

Not wanting to start an argument but could be possible that she simply felt she couldn't be a mom at that time or in general? Matter of fact I don't recall her having any plans for kids. I think even in the Olitz phase with all their talk of the house in Vermont I don't remember them putting kids in that picture. And like @preshusbabe I just don't see her confiding in anyone. Even if she has people who generally cared for her best interest, she probably uncomfortable being that vulnerable.