Despite having known about the existence of alternate universes and dealing with the effects of them as her occupation, it's still hard for Olivia to reconcile that in another place and time, there is a version of her that lives and breathes, that has a life.
And love, apparently, if the way Peter looks at her is anything to go by.
She's almost envious of herself, if that's entirely possible. It must be nice to have someone to go home to, a firm presence to count on through anything and everything.
She pictures what their life must be like in the alternative timeline; she can't help herself really. Peter doesn't talk much about her, the most obvious thing being his determination to get back to her and the other Walter, but she assumes the other Olivia must be like her—and not just in looks. There are times when she'll use a turn of phrase and Peter looks at her strangely, like he's trying to separate her and the version of her he knows and loves.
Most of the time, it's a look like he's aching.
She also knows the other version must be like herself because Peter seems to know everything about her. She remembers the first time he mentioned Rachel and Ella and the shocked step back she took at the familiarity in his voice. She shouldn't have been surprised really, that the other Olivia trusted Peter enough to let him baby-sit Ella. She can, quite vividly, imagine the scene: a version of her, Ella and Peter in her apartment playing games, laughing, loving. It's a scene that leaves behind a fleeting empty feeling in the bottom of her throat when something or someone pulls her back to her reality.
It's fleeting because Olivia reminds herself that she has a satisfying life; she has friends in Lincoln, Astrid and Walter and family in Rachel and Nina who all love her, and a job she enjoys. People would –and do- kill for what she has. She is truly happy with her life-which is more than can be said for Peter. Trapped in a world where the people he cares about don't even recognise him, much less trust him. She can't imagine how unfriendly this timeline must seem to him.
How lonely he must feel.
His Olivia must be quite something to behold. If she wasn't, she doesn't think even Peter could bear it.