I have been thinking about this for a few days since first seeing the thread. I have tried to compare how I became comfortable with the different Ts I have seen over the years.

When I was a teenager I didn't really feel like I had a choice in who I saw. I was so distressed that I knew that I needed to get help, and so trusted my T (bad experience). In college I had two Ts. The first one I also didn't feel like I had a choice. But then the second one I felt like I had a choice. For me it was a process of trusting a little more each time, especially when big issues came up. Like a lot of the others said, it was a lot of being able to say things and having my T stay calm and be supportive. With each new disclosure it was a test of whether or not my T could "take it" and understand and not reject me.

Since that first good experience with a T, I now trust my gut from the first couple of sessions if I feel like the T is going to be understanding and is able to deal with my history. If they can't deal with my history it is an automatic no-go. Then the comfortableness comes with each session. This doesn't mean that there aren't times where it is still hard to share new things.

Good luck.