Why does Nick choose to share his thoughts and feelings with Jordan in F. Scott Fitzgerald's The Great Gatsby?
Let's take a quick look at Daisy, Tom, and Gatsby first. The three of them are close for a variety of reasons. First, they are all wealthy members of West Egg and East Egg. More importantly though, they are a triangle. A love triangle. Daisy is married to Tom, but falling in love with Gatsby. Gatsby is intentionally trying to steal Daisy away from Tom. Tom is a cheating husband, but can't stand it if his wife does the same. Both Jordan and Nick are outside of that triangle. They are outsiders of that entire plot. Of course Gatsby tries to enlist Nick's help, but for the most part, Nick is somebody outside that is looking in at the situation. Jordan is also not part of the love triangle. She maintains a cold, aloof emotional distance from all of the happenings in the story. While Nick might be falling in love with Jordan, I've always got the feeling that Nick is nothing more than a passing amusement for her. Regardless, her seeming lack of care about anybody other than herself makes her somebody safe to share feelings with. Nick knows that she won't go gossiping about it anywhere else, because she doesn't care all that much about it in the first place. Nick, on the other hand, is emotionally invested, and he needs to talk out his thoughts a lot of the time. Jordan is the safest person to do that with.
Nick and Jordan have one particular quality in common - they are outsiders. Though the source of this status is different for both of them, this status is what functions as the bond between them.
Nick is a new arrival. He is not part of the world of east and west egg as Jordan, Daisy, Tom and Gatsby are (or have become). Nick remains an outsider and a Midwesterner through the action of the novel, never truly entering the world he witnesses and which forms the center of his narrative.
Jordan is impassive. She is detached. There is no world to which she truly belongs. She seems not to care enough about anyone or anything to be drawn in permanently.
This impersonal distance is what allows Jordan the status of "outsider" and what allows her to stand aloof, witnessing events much like Nick does, with detachment and without alarm.