Read this website to understand better whats going on here.

Dogs don't try to dominate either humans or other dogs. "Submissive" behaviour is not always because a dog is scared either. One of my dogs shows these as a means of trying to convince other dogs to play with her and as a simple means of showing respect. They are called calming signals and many calming signals are displayed when a dog is happy, content or excited as well as during more negative situations.

http://www.dogwelfarecampaign.org/