the original "how psychotherapy works" theory explained (best as i can)

 There is an old idea, one of the original psychological theories that still has important things to help people.  It goes back to the late 19th century.
Back then, people knew very little about how the mind and the nervous system worked.  It was all a mystery.  Women tended to get diagnosed with "female hysteria" which covered everything from what we'd call chronic depression, migraines, seizures, faintness, nervousness, sexual desire, insomnia, fluid retention, heaviness in the abdomen, muscle spasm, shortness of breath, irritability, loss of appetite for food or sex, chronic pain, loss of fine and gross motor skills and on and on.
Imagine if MS, chronic fatigue syndrome, PTSD and PMS all got shoved in one box and labeled, "Meh, who knows?"
One common theory was that the women's uterus was out of alignment, so vaginal massage and stimulation was used to try and treat it.  I'm totally serious, in the late 19th Century women went to doctor's offices to be fingered to orgasm as a cure for their physical ailments and mental issues.  The even WEIRDER thing was it often worked.
Simund Freud became the famous person he is in history because he looked at all this and essentially said, "OK, this cure shouldn't work, but it does.  Something's going on, and it isn't what we think is going on."
His theory boiled down to the idea that people have desires, thoughts, memories that they don't like to think about.  That all builds up in the mind, like water behind a dam.  When the pressure of things we don't like to think about gets too high, the dam bursts a bit.  They assumed early on that thought was like a force going through the body, and it would get "blocked" in some nerves.  Freud called the force "The Libido," from Latin libido "desire, lust, since it seemed to be connected to the sex drive somehow.
The theory was if the Libido was blocked or diverted by our thoughts, the nerves would spasm or be stimulated inappropriately, causing pain or loss of muscle control, or odd thoughts and desires etc.
The idea that Freud had was that the orgasms that other doctors were inducing by "massaging hysterical wombs" was causing the release of a lot of bottled up thoughts, desires, emotions and memories as an accidental side effect.  Being a good scientist, he looked at the idea and decided that the way to prove the theory was simple.  Find OTHER ways to get sick and unhappy people to let go of their bottled up emotions and thoughts, and see if their symptoms cleared up. 
He started off with hypnosis, then getting people to talk about themselves until they discussed what they REALLY had on their mind.  He used dream analysis, and used what he learned from dreams to get people to think out what they had bottled up and release it. Other doctors took the idea and ran with it.
And it worked.  In extreme cases, bedridden people wracked with pain were cured, blind people regained their sight, incurable allergies and asthma went away, migraines were a thing of the past... on and on.
Whole sections of the sick population were able to be treated for the first time, by getting them to let things out of their mind.
Over the years we learned this was a bit too simple, thoughts aren't a FORCE exactly and the nervous system was a lot more complicated than it looked back around World War One.  But just about every therapy that doesn't involve drugs started with Freud, and the goal is STILL to find the most efficient way to get people to unbottle what is inside that's inside and making them ill.
Stress causes muscles to tighten, hormone levels to spike, breathing to be irregular, messes with your pulse.  After days weeks or years, all this causes actual measurable damage.
Stress raises blood pressure, and the tight muscles from stress causes blood pressure to be WAY too high some places and too low others, think of a kink in a hose, or a water balloon.  THIS is a proven killer.
Some estimates are that 90% of doctor visits come from something stress related.
Everyone knows someone that doctors can't fix, they can't even name a disease.  "Maybe it's lupus, maybe it's MS, maybe it's chronic fatigue..."
Depression slows down the immune system, making us more vulnerable to diseases.
So take care of your mind!
Especially if YOU have a physical disease no one can quite figure out.  It's probably what they'd have labeled "hysteria" a century ago, and the cure has been known for a hundred years.
And if you're a sex addict, now you know why.  Orgasms can cure things, it's a proven fact.  Just learn all the other ways to unbottle your feelings so your addiction slows the hell down.
 
Links for more (and better) explanations
http://cirrie.buffalo.edu/encyclopedia/en/article/139/
http://www.instituteforchronicpain.org/common-conditions/conversion-disorder
http://www.psychologytoday.com/blog/the-mind-body-connection/201306/chronic-pain-syndrome-and-other-psychosomatic-illness
http://stress.about.com/od/stresshealth/f/psychosomatic.htm
 http://www.healthcentral.com/anxiety/c/4182/156620/hypervigilance/
 http://ptsd.about.com/od/glossary/g/hypervigilance.htm
 http://psychcentral.com/lib/the-physical-effects-of-long-term-stress/000935
 http://health.howstuffworks.com/wellness/stress-management/physical-effects-of-stress2.htm
 http://www.apa.org/helpcenter/stress-body.aspx