Do you recall what the feeling in the dream is?

It's helpful to list all your associations to empty, large public bathrooms, just brainstorming everything that they make you think of, without initially concerning yourself with the overall meaning of the dream. Chatting with your best friend in a huge bathroom ("huge" seems relevant) would be an example of an association. It might not "click," however, because you don't feel sad in the dream. But just try brainstorming all your associations.

EDIT: since you are alone in the dream, it might be saying something about the parts of your life you spend by yourself. are you introverted or extroverted? (i.e. do you recharge by spending time alone, or with people?)