I've had the same happen to me, and I said something without even realizing it would upset him; I guess there's this image that can be created with a Dom that they're totally immune to anything because they seem so strong and powerful to a submissive - but, like people have said above, they're just human, and have the same emotions and are just as readily hurt as everyone else is.

I've finally realize that, but it's sometimes hard too, they always seem so together - early on in our relationship I even found the idea of him crying strange hehe, but they're just as sensitive as everyone else, and that one of the main reason they can be so wonderful