They don't mean to offend, they're just curious. They aren't really interested in your personal condition as much as they are in service dogs and what they can do. They want to hear Lassie stories. They want to tell you their stories about dogs they've loved.
Dealing with curiosity from members of the public is part of using a service dog. Write up some answer that you feel comfortable with and memorize it so you can just spill it out as needed without having to think. If I'm in a hurry I just give a couple of sentences explaining the sorts of things service dogs do and then explain that I appreciate their interest but am in a hurry and don't have time to chat. I don't give any personal information of my own, and people have always been satisfied with that answer.
If I have time and feel like it, especially when I am approached by children, I'll do some educating about service dogs. Things like "ask before petting," "don't distract," and so on.
You can also avoid conversations by not making eye contact with strangers. If you walk into a store with your eyes held up looking at the back wall of the store and move purposefully, you are less likely to be approached because your body language is saying, "I'm busy."
You can also print off brochures, such as the one available from the Delta Society, and just hand that to them to answer their questions.
http://www.deltasociety.org/Document.Doc?id=239 (This is a two-color, three-fold brochure in PDF format)
Please note that businesses are permitted to ask about your service dog, whether you require one because of a disability and what the dog is trained to do for you. They can refuse access if you don't answer, but you certainly don't have to answer a casual bystander who is simply curious.