You never read The Story of O?
Actually, I'm being semi serious. A lot of people get off on ego deflation and some take it too far, let it guide them into toxic relationships, etc.
No, that wasn't on the required reading list when I was in school lol. I do know of it, just haven't read it.
Being in a dominant/submissive relationship is slightly different. There are two willing people agreeing, on some level, to pre-determined roles. In this case, I'd bet that both have significant insecurities that are being catered to in some way by these roles. It may not be healthy, but a situation like that could actually work.
An overblown ego is also a sign of insecurity. Those who excessively talk themselves up are usually the most insecure of all.