Sometimes you feel tired of paralanguage-monitoring and self-shrinking – it can feel tiring (for the untrained) to be always on our toes and to avoid sticking-out. Quiet people are presumably more comfortable with that but I'm not a quiet person (though I'm rather introspective or “looki”)
Sometimes you just want to, for a moment, be yourself, express (not in a loud, in-your-face way, but in a Passive way) your individuality and leave the judgement to “them”. I seem to have many family members + colleagues/bosses having that tendency, though each of them decide when to show it and when to Control it.
Humour is a decisive part of it. Some would say “No humour, don't try it.” I'm not humorous even though I find many words I speak somewhat amusing.
99% of the time I decide to “let my hair down” and speak “freely”, it has been a (conscious or semi-conscious) gamble since I have no control over the situation . I suspect a lot of times the negative reaction in the audience couldn't be completely offset by the positive. Let's face it, if there's any trace of negative reaction after you say something, it tends to last a long time, even if the positive is much more. Especially true if the negative is taken personally like a joke about age or weight. Showbiz people like to take on those sensitive topics… because they can, but it's foolhardy to “try it at home”. You also see people communicating rather directly in movies and in publications, but it's an exaggerated/distorted version of reality. In reality that kind of speak is rare, shocking. It's like playing with fire.
In my (somewhat biased) perception, I tend to see westerners as less restrained, more individualistic, speaking-for-own-self, carefree, less careful, less rule-bound… This long list of perceptions would eventually lead to the American ideal of individual “freedom”, but that word alone would be oversimplification.
Experience — In my first 5 years of working, I was often the youngest team member. I didn't have to “image-manage” myself as a future leader. I rarely had junior staff looking up to me.
 major exception is the last few days on any job.
 I suspect most of us can get used to it, just like children getting used to self-restraint once in school.