Do you ever go to a public restroom, only to find that you cannot release? If so, you are not alone my friend. In fact, it is estimated that 7% of the general population, and probably much more, suffer from what is called bashful bladder, or paruresis (Kessler, Stein, & Berglund, 1998). The psycho/physio-logical disorder is characterized by an inability to urinate in a public restroom, or in the presence of strangers(near or far). Unfortunately though, I can't tell you exactly why that is, but I will try my best to convey the current thoughts on this disorder.
However, before we get there, let's talk about how urination (peeing) works. The bladder is a large organ lined with smooth muscle(you can just think of it as muscle that is involuntary) and located in the pelvic cavity, and holds the urine brought to it from the kidneys via the ureter, as we see in the figure A. When resting, the internal sphincter (a circular, muscular structure, much like the one in your anus, except involuntary) is passively contracted, to prevent urine from leaking into the urethra (the entrance/exit path). Along with this, there is an external sphincter that is kept contracted by motor neurons under central nervous system control (brain and spinal cord).
When urinating (micturition), the nervous system causes for the contraction of the bladder, forcibly opening the internal sphincter, and through a second pathway, inhibiting the motor neurons (neurons that control skeletal muscle) that normally contract the external sphincter, leading to relaxation of the muscle. This is the basis of urination, and is the current accepted model, albeit can be gone into much more detail than what I have presented.
What role does bashful bladder play? To date, the scientific community's best understanding of the bashful bladder phenomenon is that there is a tightening of the sphincter(s) due to a nervous system response(the stimulus usually being strangers), by an unknown mechanism. It is unclear which sphincter is being contracted, or whether it is the muscles surrounding them being contracted or relaxed. What further complicates matters is that a portion of the brain which is involved in urination (micturition), the pontine micturition center, seems to be involved in the disorder, as its inhibition prevents relaxation of the internal sphincter. However, what is clear that the cause is not due to obstructions of the urinary pathways, as would happen with pathologies such as prostate cancer (Soifer et al, 2014)
Unfortunately, there is no known cure, however, as far as treatment goes, cognitive behavioral therapy, which focuses on changing the thought processes that are behind people's difficulties, has been shown to be an effective treatment option for managing symptoms (Soifer et al, 2008). For instance, thinking to yourself how absurd it is to think that someone plans to harm or disrupt you, or having a friend gradually come closer and closer while urinating may help relieve some of the symptoms of bashful bladder. However, this may not have always been so absurd, because one of the theories as to why some people have this disorder, is that it is a remnant of our evolution. That is to say, when we were once both predator and prey, one wouldn't want to begin to urinate when a predator is around, as it leaves you both traceable and exposed. Coming back to the topic though, there are also surgical options, as well as some trial drugs. The surgery is more of a last resort for very serious case of bashful bladder, and the drugs (alpha/beta blockers[blocks nervous system receptors], botox [the same kind they use for facial relaxation]), by-and-large, have not been clinically tested, and so most data on them is anecdotal and have had limited success. With that said, much of that data can be found on this website, paruresis.org, a support site made for those with bashful bladder. To that thought, share this article, as it could help out somebody you know!
Soifer, S., Nicaise, G., Chancellor, M., & Gordon, D.2009). Paruresis or Shy Bladder Syndrome: An Unknown Urologic Malady?. Urologic Nursing, 29(2), 87-93.
C., Stein, M. B., & Berglund, P. (1998). Social phobia subtypes in the National Comorbidity Survey. American Journal of Psychiatry, (5). 613.