From www.drbenkim.com
Root Causes of Anal Fissures
The most obvious cause of an anal fissure is direct trauma to the anal canal. Childbirth, anal intercourse, and insertion of any foreign bodies into the anal canal can cause a fissure.
Chronic constipation and chronic diarrhea can also cause an anal fissure by repeatedly straining the lining of the anus.
In the vast majority of cases, an underlying cause is chronic tension in a muscular ring - called the internal anal sphincter - that surrounds the anal canal.
If your internal anal sphincter is chronically tense, blood flow to this region is reduced. Reduced blood flow causes the lining of your anus to become more susceptible to tearing. Reduced blood flow to your anus also makes it harder for a fissure to heal.
This is why some people tear relatively easily when they try to pass hard stools, while others don't develop a fissure even when chronically constipated - the tone of your internal anal sphincter largely determines if an anal fissure will develop when your anal canal is excessively stretched.
So what can cause your internal anal sphincter to be chronically tense? In the absence of overt neurological dysfunction, the most common cause of a hypertonic internal anal sphincter is ongoing emotional stress.
Emotional stress causes your autonomic nervous system to gear up to fight or run for your life. One of the consequences of being chronically amped for a fight or flight response is a tense and dysfunctional gastrointestinal tract, which includes a taut internal anal sphincter.
Without exception, every person that has come to me looking for a way to heal a chronic anal fissure has reported experiencing significant emotional stress around the time that the anal fissure first appeared.
My experience has been that botox injections, lateral internal sphincterotomy, application of nitroglycerin ointment, and other conventional medical treatments for a chronic anal fissure tend to lead to temporary healing at best and a re-occurrence of the fissure if emotional stress continues to take its toll on the internal anal sphincter.

I can relate to that ..when I'm stressed I tend to hold it down there

When I had a fissure (pre-LIS OP) I certainly found that stress increased the pain. Not surprising I suppose, as so much of the pain with anal fissure concerns muscle spasm and muscle tone in the anal sphincter. The CRS I saw confirmed that becoming anxious or upset increases the pain - with me this was only for the short period when i was feeling upset about something - if I'd had an argument with someone for instance.
Two of the surgeons I saw said that there's a personality type who tend more to fissures - highly ambitious, tightly coiled people emotionally - the sort who may be Wall street or City bankers. The thing is, I couldn't identify as this type of person, so I suspect this is only a general trend and doesn't cover everyone.
The second surgeon I saw quoted studies that show that anxious people tend more to fissures. I found this particularly unhelpful, as I found that having a chronic fissure was the source of my anxiety. During my psychology undergraduate degree a few years ago, I participated in lots of research and took anxiety inventories. I always scored as very low on anxiety! I thought, therefore, that they may be mixing up cause and effect. Have they ever considered how stressful it is having a chronic fissure, and how that can turn a fairly relaxed person into an anxious one?
In the article above, I find the following phrase particularly unhelpful:

My experience has been that botox injections, lateral internal sphincterotomy, application of nitroglycerin ointment, and other conventional medical treatments for a chronic anal fissure tend to lead to temporary healing at best and a re-occurrence of the fissure if emotional stress continues to take its toll on the internal anal sphincter.

The lateral internal sphincterotomy has something like a 95% success rate. How can emotional stress take its toll on the internal anal sphincter if the muscle's been cut into? The muscle isn't capable of responding in the old way.
I'm extremely suspicious of things I read on the internet, having been plunged into despair more than once by reading something during the days pre-op when I thought I'd never be better. The author of the above article identifies as a chiropractor and acupuncturist. He's not a medical doctor so far as I can see, and not a colorectal surgeon specialising in anal fissure and other disorders of the lower bowel. To me the above phrase just sets people up to think that there's no permanent cure from conventional medicine and they need his treatment! My CRS said that I won't get another chronic fissure now that I've had the LIS and that anything that does occur will heal rapidly. Since my anus settled down to normal after the surgery, I've only once had something that caused pain and made me think I'd got another fissure and it was when I was on holiday, it was very hot (another factor related to development of fissures - due to dehydration) and I'd had a changed diet. But by the following day I was fine again! I'd say that constipation and diet were more a factor in the development of my original fissure (and yes, the weather was very hot when I developed my original fissure) and changed diet resulting in larger, dryer stools were a factor when on holiday.
If emotional stress is the underlying cause of fissures in the majority of people, as claimed, then why isn't another raft of treatment for anal fissure to see a therapist or psychologist?
I think I'd rather trust the opinion of my CRS who really does appear to have read all the research regarding treatment outcomes.