How many years of your life are you going to sacrifice for someone else’s future?

That’s the critical line and question for Beckett. She became a cop to help the families of murder victims not have to live with the pain of not knowing why their loved ones died. This new job would mean dealing with corporate cover-ups, terrorist plots, and more, with her potentially being able to save thousands of lives, at the expense of her own. She did that when she became a cop, and if not for Castle she’d still be in that mode. As she walks through the precinct, her home, she makes up her mind to not take the job and risk things with Castle. It’s why she can smile at the guys when she says she has something to tell them, but she has to tell someone else first. If you have any doubts about that, just look at how relaxed and smiling she is when she sees him. This is not a woman walking to go break someone’s heart.