For me at least, I think if I were given time and retrospect, I could figure out why (or if it was very obvious that the other person was going to make me feel worse or generally had a history of helping me talk through things well), but I have sometimes opened up to people I don't know well, and other times to people I do know well. Initially, I'd be hard-pressed to say exactly why with either (unless it's based on my past history with them). It's not a vote of confidence/lack of confidence in the person, as there are many people I value and whose strengths I need, but not in that particular role. It's more that at the time, the person seems to answer something in me that I don't always even know is there till we start talking. I don't usually think, "Now, I need someone to process this with. I wonder who it is that I'll talk to".
One of the difficult things about being somewhat unaware of which feelings to give the most weight to and of thinking I'm doing better than I am sometimes is that I very occasionally will react quite emotionally when my sadness is still unprocessed and I am not just double checking courses of action or discussing solutions, or are dissecting the experience after the fact to see what I can take from it, etc. In the few cases where I have reacted really emotionally just because of uncontrolled overflow, it can surprise me just as much as the other person and it is terribly embarrassing. When that happens with someone who either tries to rationalize it away for me, or wants to immerse themselves in my experience to empathize, I find myself getting very frustrated with them, which further adds to the load. So for myself and their sake, I really don't like to do those earlier stages with anyone, till I feel more ready to discuss where to go from there.
Last summer, my mum was under a lot of emotional stress, which resulted in her acting uncharacteristically. Because I was generally the one she could always count on, her behaviour with me seemed to be different than that towards other people and I read it that she had less faith in my skills or competence. I was out with my (unofficial) sister who was visiting for the summer and who had spent many of her growing up years in our house, so she understood the dynamics. I thought I was doing fine, and then as she and I started talking, I said something offhanded about what was going on, and suddenly realized it hadn't come out offhanded and I was doing the heaving sobs thing, which I almost never have done even when I am alone. She was being empathetic, but started rubbing my leg sympathetically and I remember just bursting out, "Don't rub my leg! I'm going to be fine." I wanted to get back to normal as soon as possible because it wasn't really at a stage it could be discussed yet. It's like trying to get to a bathroom on time, thinking that you've made it okay and suddenly it just can't be held back and it becomes apparent that what you thought was a stomach gurgle was actually violent diarrhea. (Sorry for the unappealing and graphic picture). Unless someone is bringing you new clothes at that time and giving you time to clean up alone, you're not going to like it. If they try to open the bathroom door, or discuss the incident while you're still cleaning up, their efforts are going to be counterproductive. I guess in that sense, you could say that I make a choice of who not to talk to, but that doesn't explain how I determine who to talk to later on.