Answering the big questions:
How do people go about ruining their vision?
Why would someone ruin their own vision?


Preface
This is just a brief outline of what I've been through. Like I said, I was a worst case scenario. You will see what I mean by that as you read on. Bottom line - Since even I was able to correct my vision, it's extremely likely that anyone can.
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I used to think that vision was very delicate, and easily lost through misuse. As it turns out, the eyes have incredible resiliency, and a healthy person has to work long and hard at disrupting them before the vision even begins to fade. On the most basic level, it's chronic tension that is the underlying cause of refractive errors. I have reasons to believe that I started building this tension at a very young age. I increased the rate of this process dramatically around age 7. Yet amazingly, I didn't begin to need corrective lenses until around age 17.

I disliked the feeling of wearing glasses so much, that I immediately discovered and started following the teachings of Dr. William H. Bates. He was the first to establish and prove how the eye actually functions and how to correct errors of refraction. After publishing his results, he was expelled from the college where he had been doing this research. I decided to "give his techniques a 2 week trial". After just 2 days of performing his simple techniques, it was obvious that it worked, and worked very well. However, other factors were at play that made a lasting improvement impossible for me. These were the exact factors that had caused me to loose my perfect sight in the first place. I wouldn't figure out what these factors were for about 2 decades.

I spent those years getting worse and worse, occasionally getting a stronger prescription. Each time I would see amazingly well for perhaps a week, then I would fall back into a comfortable level of visual acuity that was just slightly less than normal, under ideal conditions. I eventually achieved a correction of 5 diopters. What the means, basically, is that I was pretty much completely blind. 5 diopters is what is considered to be "legally blind". It's when taking off my glasses caused the whole world to become nothing but a soft shading of different hues of color. I should mention at this point that I was nearsighted, and so had a (-) correction, and could see well up close without correction. This is what it looks like in the eye.

At a point, even seeing up close was becoming difficult. Wearing glasses or contacts made reading or other up close work very uncomfortable, so I did my close work without lenses. I got to the point where I could only see at the distance of 4 inches when going without correction. To read a newspaper for example,  I had to lay it out and crawl on top of it, then scan each line from end to end. Eventually, even my close vision was beginning to fade. Sometimes, I would get tired of glasses and would switch to contacts. Then when I would become tired of the contacts, I would switch back to glasses.
One day, I reached a state of desperation. I just decided that I was willing to do whatever it took, for however long it took. I removed my glasses and decided to just not wear them anymore. That act, the desperate act of deciding that I was willing to change, caused me to quit defending who I thought I was, and embark on a journey of self-exploration that would turn out to be far more beneficial than I could imagine. The first thing that happened was that my chronic throat clearing problem abruptly went away. I had worked for many years with people who had the same throat clearing problem. I would buy slippery elm bark lousenges for me and my friends. They were a great relief, but didn't address the underlying problem of tension in the throat. My vision was improving by the day, but I started to realize that there were some conditions that were holding me back. For one thing, I started closely studying my neck, shoulders and upper back and discovered a tremendous amount of buried tension. This tension had hardened the muscles and nerves a great deal, and made them tender and painful when pressure was applied. So I applied pressure! 
It's been said that people who are nearsighted tend to hold tension in the back of their neck. And that people who are farsighted tend to hold tension in the sides of their neck. Well, I was holding tension in the back, on the sides, and even the front of my neck. The tension didn't stop there, it had basically filled my entire body. Much of it was easy to let go of, and I regained some lost flexibility very quickly. However, the areas of my neck, shoulders, and upper back had become severely effected from long-term chronic tension.  It alters the posture in subtle ways, and I have become so intimate with this work that I can see these same effects to varying degrees in most of the general population.

Once I started doing deep tissue massage on these troubled areas, I discovered that not only had some muscles and nerves become as hard as concrete, but that this 'concrete' held clues as to what kind of tension was being held there! In other words, as I broke these structures down, I would be taken right back to the emotions that were prevailing at the time of their formation. This was huge, because those emotions provided the means to achieving the psychological changes that were necessary to stop this whole process. Put another way, since the tissues were now mobile and hydrated, they could no longer store these painful, suppressed emotions, and thus I was forced to deal with them directly, immediately, and permanently.

So I got to relive much of my life, in a very painful and unpleasant way. All the while, becoming more limber, softer, and with an enhanced sensation of "life" in these tissues. My vision quality directly tracked these changes, and improved a bit every single day. About 2 years ago, I began occasionally testing out at 20/15 on the eye chart which is "better than normal", it's reading the smallest line on the chart in the distance.
So that is a short answer to the "how someone ruins their vision". It ignores all the details about exactly what tension actually is and how it effects muscles and nerves, including the eye muscles. As for the "why people do it", I would imagine that the answer is deeply personal to each individual who has subconsciously chosen to do this to themselves. I got to find out what my reasons were, and in the process enjoyed many interesting side effects and benefits. For example, I found the answers to riddles such as:

Why is it that time seems to accelerate as you age?
Why is is that things looked bigger when you were young?
What is chi energy, and how does it work?
How does tension effect hearing and memory?
What causes people to be pigeon toed?
What causes scoliosis (twisted spine) and how is it removed?
Why do people use their left or right brain at the expense of the other half?
What causes cold hands and feet?
How do you stop clenching and grinding your teeth at night?
Why is it important to maintain your flexibility?
.....and many more.

In addition to encountering enigmas and discovering interesting paradoxes
that relate to eyesight and vision improvement,
I also spent a lot of time
exploring the infinite qualities of:

Time & Space
Vision
Relaxation
Consciousness

Sometimes, while deep into this healing work, I would suddenly feel the need to write down observations and ideas. While reviewing my notes to put together the list above, I came across this scribbled statement:

The smarter you are......
The more you realize your ignorance,
The more you realize that you can learn from others,
The more you accept their differences,
The less you try to change them,
Which allows you to be....

To be....what??? Is that all, or is there more??
That's what I hate about cryptic old messages scribbled among dusty old notes....