We all wonder "Why?" an "How?".  My personal view is that most of us have symptoms of some sort that are IC-related from early childhood. For me, my childhood was extremely healthy - very healthy food, lot of exercise, wonderful home life etc.. Yet, I had symptoms of inflammation in my bladder (very brief) from the age of 9/10 (perhaps even earlier). This would last a few seconds, then disappear for years (I didn't know what this was, of course, but looking back I can remember one or two of these 'episodes'). The females in my maternal line also had bladder issues (frequent cystitis, unexplained non-bacterial cystitis) which never fully developed into IC. Thus, the genes are there. Retrospectively, I have been dealing with bladder-related issues (thus, IC) all my life, but it has behaved in such a way as for me not to have realised that it was one and the same disease. It has come and gone (my latest bout has appeard after a twelve year remission) with no rhyme or reason. It has never debilitated me before now.  I have never noticed it coinciding with trauma, food, antibiotics etc.. My latest bout came on in pregnancy (but then my earlier pregnancies are what made it disappear in the first place). Did the pregnancy (i.e., hormonal trauma) cause it? Do antibiotics cause it? Do menopausal changes cause it? Do stressful life events cause it?  I think not. Antibiotics, pregnancy (hormones), pelvic surgery, stressful events are clearly triggers in vulnerable individuals - but to attribute causality would be very difficult. As MissCC mentions, millions of people take antibiotics, birth control, eat badly, have traumatic events happen in their lives, etc.. but do not get IC. That is not to say that these are not causes in US - the vulnerable to IC population - but most likely we are simply predisposed, and then a trigger works against us ( correlation does not imply causation anyway).  My personal view is that we are vulnerable, genetically, and when we come across an environmental trigger, we get hit by it. It's the old genes x environment equation. Why did I have bladder symptoms even before I ever took an antbitotic, or any medication, before puberty, before pregnancy, before stressful life events,  when my food, home life etc. were great? Where did that come from? However, this is not to imply that we can't recover. We most certainly can.
I still don't understand this whole thing - in the past twelve years (when I didn't even know I had IC - I was never diagnosed to begin with), I didn't take medication, ate an extremely clean and healthy diet (I drank one bottle of organic cider some Fridays - that was the extent of my 'badness'; oh, and I have never had a sweet tooth - ever - so would always rather have another bowl of homemade soup rather than anything sweet - it has always been like this. I was never allowed chocolate before 4 years of age!), exercised, was extremely happy in my home life/work life, no stress (never prone to anxiety or depression etc.), was healthy, never went to the doctor and never took medication, had a healthy pregnancy, and I STILL got IC. Personally, I am finding it easier to accept if I let go of the search for causality. There will never be an adequate explanation for why we got it and others who lead seemingly (and this is important - trauma to one may not be trauma to another) worse lifestyles didn't. I am (was) a scientist so letting go of the search for reasons is very difficult - but I honestly now think that life is just random. We think we can control life events but really we can't. Having worked with statistics during my career, we can find associations between anything - and most of these can be meaningful or spurious, depending on how you want to interpret them, eg, we can find studies on the asscoiation between trauma and IC and STILL this won't explain why YOU got it.  My take on all of this has been to try to ignore this search - we are vulnerable, we got it, we can get better. We can all get better - much better - but we will never know how or why we got it. I think that Matia is correct in that the proximal cause for IC is yeast/microbial overgrowth - and this is why we can ALL get better - because there is a mechanism to this disease that we can TREAT - but what makes US, the vulnerable IC population, predisposed to this overgrowth in the first place is unclear (the causality is unclear. The triggers are perhaps more clearly delineated. But I really do now think that this is unimportant).