Diana adores Peter - it's obvious, and if Peter weren't male (and married), Christie would be jealous and insecure.
Well, insecure. She's already jealous. Peter (or rather, Diana's job) gets more hours of Diana's time than Christie does. It's not that it makes her feel insignificant. But it rankles.
That, and the fact that Diana wants to live in New York for Peter more than she wants to live in DC for her girlfriend.
Christie had more friends in DC, and it was close to her family too. Her job there was as top-notch as the Manhattan hospital where she now works but a little less cutthroat in terms of the other doctors. It's not that Christie expected the warm fuzzies at work - collegiality among young doctors was always dappled with hints of oneupmanship, but at GW it just felt like the quality of the competition didn't correlate quite so directly with how viciously competitive they were. (The patients, and the people who put them there, were the same level of cutthroat at both places).
But Diana didn't like DC. She didn't like the humidity or the fact that it was impossible to get a cab, or the fact that they couldn't go out to dinner without running into someone who also works in government. Or worse, lobbyists - Diana always managed to get into (and win) the occasional snark-off with lobbyists who had the ill fortune to frivolously throw around their plastic for all to see.
Of course, Diana would have put up with all of that - as well as missing what she liked about New York - if Peter were stationed in DC. Christie knows this.
She also knows it's not JUST about Peter. It's not just his friendship or mentorship, or the way he thinks the world of Diana (and who in their right mind wouldn't, Christie thinks). But at the DC office, Diana was miserable. She dreamed her whole life of being in law enforcement, of taking down the people who think their power means they can do whatever they want. And in the DC office, Diana learned that sometimes large government bureaucracies aren't actually created specifically to give their agents the chance to be heroes.
All the cases in DC were about power. The straight up raw political kind. And in DC, power is all about perception, so every case, every move, was about how it would look to various groups who were important enough to tank a case if they really wanted to. Not to mention that her boss in DC seemed to take Diana's intelligence and wit as a threat rather than an asset. He was mostly of the mind that women were an asset to the Bureau because of their superior social skills. He also cared more about using his position to get a private sector job eventually than actually reeling in any of the big fish.
So Diana was ignored, patronized, and given little challenging work, and most of that was at a desk. And when she actually did get the chance to nail someone who deserved it, she found that justice and proof and the badge didn't mean as much as who had lunch with which senator and who played golf with the Attorney General's scheduler. And so Diana was left to choose between becoming the kind of person who protected those in power and snared the rest, or to simply resign herself to not excelling at her job.
Neither was a real possibility.
And then Diana, the entire time, would remember what it was like working for Peter Burke. The head of the division with the highest success rate, filled with people hand-picked by Peter for their integrity and intelligence, all of whom Diana liked working with, all of whom thought passion and toughness and cleverness were things to value rather than things to undermine.
And of course Peter didn't care who he angered or how well connected they were; the only thing that mattered in an investigation is what you can do and what you can prove. And Peter believed in Diana, felt deep affection for her, yes, but was still happy to give her - the youngest agent in the division - some of the toughest (and most dangerous) assignments, confident in her skills and her smarts.
The contrast with working for Peter made working at the DC office far worse for Diana. Christie hated watching it, seeing Diana's hopes and dreams and even her self-confidence struggle to survive the corruption and disregard.
There were times when it was so bad that Diana talked about quitting for another line of work. On those times, Christie didn't respond with the first thought that popped into her head. She instead did her best to figure out what to say, what Diana needed to hear. She would say things that were understanding and encouraging and make it clear that she would support it no matter what Diana decided. She always felt guilty after, wondering if maybe she hadn't quite done it, if maybe Diana needed something else from her, maybe needed the "Go back to the Bureau and kick their asses" speech more than she needed the "Whatever you want is fine, hon" speech.
When Diana finally decided she couldn't abandon her badge - it was too much part of who she was - she asked Christie to move to New York, to try living there again. Christie wanted desperately to say no, to give some reason that it was asking too much to move to New York but it hadn't been too much to ask to move to DC. But eventually, it was probably her guilt than made her agree. Christie was vaguely disgusted with herself for wanting Diana to be less than she was.
Peter, Christie thought in a moment of slight bitterness, had never asked Diana to be less than she is.
So they move to New York. Christie misses DC but does her best to adjust. She makes friends with Diana's friends. She gets used to Diana being gone in the middle of the night because Peter needs her for some secret work that she can't even tell the Bureau about. (For Peter, Diana would risk having to give up her badge).
The assignments get more interesting (more dangerous). Diana is tough, Christie knows, but she can always tell when they're about to do something dangerous by the way Diana kisses her goodbye in the morning. Diana dealt with loss early, and on the days when she anticipates a gun being pointed at her, it's never a quick kiss. Diana always lingers just slightly to look right into Christie's eyes and smile. It means that Diana is trying not to show that there's a worry at the back of her mind that she might never see her girlfriend again.
Diana has no idea that Christie knows this.
Sometimes Diana comes home after a tough day and says nothing about it, just collapses on the bed still dressed, puts her head in Christie's lap, and thanks her for being perfect.
Sometimes, Diana has a really good day and comes home grinning; she can't wait to tell Christie the story of the idiot who thought he could pull a gun faster than her and ended up with his nose smashed into the sidewalk, or the story of the guy who thought a ridiculously slow right hook had a chance of making contact with Diana's face. Christie would smile and shake her head, wondering how she found this superhero of a girlfriend. She certainly never shows how much she wishes Diana ran from rather than toward danger. Like a sane person would.
Sometimes Diana says that she loves that Christie is a doctor. It shows that they both want to make a difference - they don't care how hard the job is as long as they're doing something significant. Christie never quite manages to tell Diana that she has never felt as insignicant as she does when she steps up to an operating table knowing full well that the most she can hope to do is to be able to honestly tell the loved ones that she did everything she could. It's a feeling she has often, as neural trauma patients come in and out, a neverending cycle of violence and pain that Christie knows would certainly continue with or without her.
She also certainly never tells Diana that, more than anything, she wishes Diana would quit her job. That Diana would decide that maybe it's not so important to be the most clever investigator, or to take down this week's bad guy so that the next bad guy can rise in the ranks and await being taken down himself, the neverending stream of hurt and exploitation never even taking a break.
She never suggests that maybe Diana doesn't have to be a knight in shining armor (maybe Diana doesn't have to be Peter Burke).
Christie does her best to sound supportive and understanding. She tries to care as much as Diana does about Diana's dreams and principles and ability to USE her amazing talents. So she never tells Diana any of this. She never tells Diana that she has operated on hundreds of gunshot wounds to the head, and that while her survival rates are 20% above the average surgeon's for that that kind of trauma, they're still below half.
She never tells Diana that exactly 54 of those gunshot victims were in law enforcement or that only 28 survived more than a few days.
She never tells Diana that she has long stopped believing that if you're smart and careful and have a will of steel, you can do anything you set your mind to. Sometimes the bullet's lodged too deep. Sometimes, they get to the table too late. Sometimes - and it only has to be once - the cocky criminal really is a faster draw.
Christie has met Peter Burke only a few times. He was kind and engaging and made it clear he thought the world of Diana. Once, Christie thanked Peter for making it possible for Diana to come back to the White Collar unit. Peter answered with a big, proud smile, and said that Diana would have a job any time she wanted as long as he worked for the Bureau. Christie smiled politely, pretended to have a headache, and asked him to tell Diana that she didn't feel well and would be going home early.