In your hypothetical situation, I can see where some people would be tempted to think that sharing their fear of Hawaii would be good. In my own, personal experience, it would be a fairly stupid idea, under the circumstances. To begin with, I'm aware that "most people" don't HAVE a fear of Hawaii and are going find MY fear to be irrational because they don't understand it. If I tell them of my fear, 99 chances out of 100 they will tell me some version of "You're being silly, get over it." They MAY even tell me that I DON'T have a fear of Hawaii, which is also a bit hard to hear. No good comes of that. Besides that, this fear is MY problem and I'm the one who has to deal with it. They want this party? Who am I to deprive them of the type of party they want? Just because I'm a nut case and find the idea terrifying does give ME the right to ask them to change their plans. And, maybe they're super nice people and will feel sorry for me and want to change the nature of the party on my account. That would be HORRIBLE! Then I'm responsible for them not getting to do what they wanted. I'm going to OWE them. At some point, I'm going to have to PAY for this! I KNOW I will, that's the way it ALWAYS goes and it will be bad......
Does that clarify anything? What's kind of funny and maybe a little alarming is that I started off planning to give a calm, rational explanation and was actually kind of getting into it by the end.
This isn't hard and fast, and therapy and where you're at with things makes a difference, I suppose. Personally, my experience is that anything I say can and will be used against me, so I'm best off keeping my feelings (such as they are) to myself. Like everyone else has said, there are a lot of variables and there isn't one answer. And the answers are going to vary some with the person. I would think some guys (in particular) would have trouble sharing their fears because the stereotype is that guys either don't get to BE scared or they "deal with it".
One thing I know absolutely for sure, just because someone wants me to talk about something or share something that doesn't mean it's a good idea. And just because someone says they can handle it doesn't mean they can. And 100% for sure, just because they say "You can trust me!" doesn't mean you can. (In fact, I think it usually means you can't.)