Along with having been a part of a society for over 20 years, Dennis's existence might also explain why so many people in the films have such a non-reaction when meeting Ted. Toys coming to life is not just relegated to him, so Ted's existence just isn’t as special anymore.
It also lends more credibility to John's idea at the end of the second movie that the teddy bear he gifted to Ted and Tammy-Lynn's adopted son could be brought to life with a magical wish.
This could be why when Ted and John are in disguise for their prank on Clive at a restaurant, Clive doesn't see anything strange about a living teddy bear; he assumed it was a different teddy bear that came to life.
