Here's a piece of a paragraph about being totally vulnerable with someone from a David Foster Wallace short story. The protagonist – Schmidt – is briefing a focus group:
… Schmidt had a quick vision of them all in the conference room as like icebergs and/or floes, only the sharp caps showing, unknowing and -knowable to one another, and he imagined that it was only in marriage (and a good marriage, not the decorous dance of loneliness he'd watched his mother and father do for seventeen years but rather true conjugal intimacy) that partners allowed each other to see below the berg's cap's public mask and consented to be truly known, maybe even to the extent of not only letting the partner see the repulsive nest of moles under their left arm or the way after any sort of cold or viral infection the toenails on both feet turned a weird deep yellow for several weeks but even perhaps every once in a while sobbing in each other's arms late at night and pouring out the most ghastly private fears and thoughts of failure and impotence and terrible and thoroughgoing smallness …
Schmidt / Wallace says this type of intimacy and openness and display of vulnerability can only transpire in a good marriage. Whether it's a marriage or a very special friendship, it's clear only the most intimate relationships can play host to a person's confessional, private fears. Forming these relationships is hard: Some people never are able to find a person to whom they can be vulnerable in the way Wallace mentions (even if they're married).
I've blogged in the past about two related concepts. First, I've wondered whether you can be truly honest with anyone in your life other than a paid professional therapist. Other than some outside professional who's paid to listen to you, every other person in your life, no matter how close you are to them, has an agenda and bias and you censor yourself to respond to that agenda. Second, I've noted that the concept of a singular "best friend" seems limiting — the idea that one person can fulfill most of your emotional needs. Rather, the "composite best friend" means you have a handful of people who are close to you who stimulate you and engage you in different ways.
Bottom Line: Most people crave intimacy and an opportunity to share all their deep dark private insecurities. Those who can fulfill this need through a single soul mate or through close friends I think overall have a richer life. Cocooning against the world is not, really, a sustainable long-term position.