Actually, this happened to me a few months ago. There's definitely an air of fear of rejection. I was friends with this guy(esfp) for over a year or so, and it was the same situation. He loved hanging out with me, but one day he looked at me and said "I feel like you know way more about me than I do about you." I just kind of sat there and stared at him. One time he forced me to share something personal with him and I was pretty upset for the rest of the night, and he noticed and felt bad about it.
Ultimately, I eventually needed some time alone because I wasn't feeling right, and he kind of took it personally. Usually I was very happy and outgoing when I was with him, but I needed some time away from people to recharge, and he didn't really accept that. So, I decided that I would sit him down and have a talk with him. I really opened up to him that day. I wanted to explain that I'm not always a ball of sunshine, and why that was.
Unfortunately, at the time, this still didn't satisfy him, and he ended up throwing it in my face. I felt like I was being extremely vulnerable, and it hurt so badly to have that spit on.
Needless to say, I've been ignoring him ever since. He wants to be friends again very badly, but I just can't look at him the same way after what happened.
So, to get to the point, yes, it's very difficult for ENFPs to open up, but it's for good reason. We're sensitive people, and all we're doing is taking steps to protect ourselves. We don't run around to everyone spilling our true life stories for all to hear. To an extent, it's a good thing.