Alright, I will try. I will try to explain myself during those moments I have had as a child, as a teen, and as a young adult. Maybe it will aid in clarifying the fog for that other person. Maybe it will help you understand why your spouse, child, friend, grandson is thinking these thoughts. Enough with the maybe's already, I won't know until I try:

"It's my fault that mom cries so much..." "It's my fault that we can't go anywhere fun this summer.." "It's my fault that dad has to work so much.." "It's my fault that we are taking so long.." "It's my fault dad has trouble lifting me and the cast.."
Why did I do this? Why did I think these thoughts? Because it was, and is, so easy. It happens with such speed and ease that when I really think about what just crossed my mind - it is a little disturbing I can get there that quickly.

When I was a child it was the simple explanation that would morph into some uglier thing in my head. The simple explanation was usually something like, "it's not safe for you to do this..." and of course somewhere in my screaming tantrums that translated to: "you have brittle bones and the consequence is you cannot do this." That wasn't the reality of the situation at all! O.I. was never used as a negative consequence when I was growing-up - it was never presented to me that way. However, in the world-view of a child everything is black-and-white; my perception of reality was clear-cut and concrete, I had no ability to think about anything other than what was five feet in front of me. So began the phase of "it's not fair!" 
Recognizing that something "is not fair!" essentially acknowledges that there is an alternative that would be preferable; however, that alternative is not attainable or allowable. In other words these were my teen years. For any thing I was not allowed to do my brain schemed of ways to do it anyway, just more hidden or away from prying eyes. My teen years were riddled with alternative-half-cracked solutions I jimmied up. But I did some of those things because it was my way of not having to deal with the reality; I refused to accept any part of me that prevented me from doing what everyone else did. In my mind I thought that if I did accept myself, I would be facing some alien I was too cool to give the time of day to. I didn't realize that facing myself wouldn't make who I was my fault. Whatever foresight my tunnel-vision managed to obscure did not let me see that my differences did not exist in order to be blamed.
Through some series of events and life experiences I got tired of playing Bugs Bunny. I got tired of always trying to escape from the traps, from being the clown with a trick or twelve up her sleeves - always at the ready to slip away. So I have begun to take the time to see what the heck is in the mirror when I look at my reflection. Some days it is easier to do than others; some days I am lazy and would rather not take the effort to grapple with what's in front of me. Some days I still mumble "it's not fair, if only I weren't..." and I get this immediate sense of coziness, because that is what I grew-up saying for so many years. It's cozy not because it 'fits' with how I see myself now, but it is cozy because that was a saying throughout my childhood - there is a great familiarity for me in that mentality. It's still so easy to do.

In between all of this, of course I had family, friends, teachers, etc. who said "this isn't your fault.." "don't think like that, you didn't ask for this.." "don't blame yourself, you can't help the way things are.."  And usually I would respond with a tireless huff and a long drawn out "I KNOooooowww-uhhh" like a foghorn warning away the obvious.. because I did in fact know I was not to blame; I knew that the explanation was much more complicated than that, and that it had to do with making babies, genetics, and other weird things.
But just as I eventually grew-up and learned (am still learning) how to accept/manage the things I get thrown, it might be helpful for someone out there -- if the individuals in that person's life... also learned to accept the reality of these thoughts. Because chances are that they will be thought regardless of what we are told by our loved ones. Acknowledging that such thoughts do wander in and out during various life stages just might help that person try a little less fleeing, and try a little more growing. Now it's your turn: You won't know until you try.