Here's the multiple choice answers to the question as to why people wear sunglasses indoors: 1. you have no other convenient way to carry them and don't want to lose or damage the sunglasses. 2. you can't see or have some kind of medical condition that requires the sunglasses. 3. you are a fugitive and need a quick convenient way to disguise your identity. 4. you are famous and need a quick convenient way to disguise your identity (not sure if this ever works). 5. you think it looks cool and/or you want to emulate your favorite rock star or actor. Personally I have used #1 excuse occasionally, but this excuse is only valid for short periods of time, like running into Walgreens or something. If your answer was 5, you are wrong and we have all figured out that you are neither cool nor famous.
I think people who wear sunglasses indoors are just trying to draw attention to themselves, even more so if they've got a dog.