Hm.
Well, the ISTJs I know don't suffer from chronic pain; closest I can think of is that one of them is prone towards ear infections -- but then again my ESFP friend gets pneumonia every year and I think the level of misery in each of those is comparable. So I dunno about correlations between those things and type. If I end up having chronic pain it'll be from Rheumatoid Arthritis, which I will have gotten from my INFJ mother.
As for pessimism: that makes sense. Not that ISTJs are pessimistic -- they would probably say that they're realists -- but both INTJs and ISTJs often come across as that, to me. Cynics. And I probably come across that way too, to a lesser extent. I think it has to do with having high standards, and being all too aware that most things don't fit those high standards. If the ISTJ you know was remembering his childhood negatively to you, the thought process was definitely "I know what a happy childhood should be, and my childhood didn't fit those criteria -- therefore it was an unhappy childhood". And if the ISTJ comes across as cynical/pessimistic regarding anything else, it'll be from the same thought process: "From my experience, this doesn't work out the way it hypothetically should, so it probably won't this time either." Si to the max.