Everyone thinks they're a great game designer. Even if they have no experience. Even if their idea of a great game is "the game I want to play, even though no one else will want to." If you tell people you're a game designer, one of the two typical reactions is "hey, I have this great idea for a game..." (the other reaction is "what's a game designer?"). Basically, it doesn't matter if you're a Junior Designer on your first gig or if you've got twenty years experience; everyone you meet thinks they're a better designer than you. This is something you get used to pretty quickly.
Strangely, I didn't see this much when I became a teacher. I don't think I ever had a single student who felt that they knew more about game design than I did. The student/teacher social dynamic is apparently stronger than the "everyone's a game designer" thought process. I have no explanation for this. (It's not just that I have the power of assigning grades; students who didn't even take my classes treated me with a respect that I'm totally not used to.)