I agree with adzymcfadzy, as several others have - sometimes a bit of both (motivating sometimes, depressing and frustrating other times). I don't think insecurity in itself is good for a career, but I do think that what a lot of people call insecurity - even with reference to themselves - is really just healthy humility. DBAs, at least as much as other IT workers, can benefit greatly from healthy humility.
I think I mostly have the healthy humility part mastered, but I have to be honest and say that feeling insecurity or "unhealthy" humility is very easy regarding SQL Server specifically, and databases in general. It's almost as if you can tell that if anyone says databases are easy, they really don't have a healthy respect for just how hard database administration is.
I constantly wrestle with different ways to increase my SQL Server knowledge, and I rarely feel that I am learning enough, fast enough. And I sympathize greatly with anyone else out there who feels like they are always behind the curve. SQL Server is a gargantuan topic. A lot of days that is also what makes it the most fun in the world. It's like that room in Raiders of the Lost Ark that keeps going, and going, and going. Never boring, that's for sure.
Thanks for this question, and thanks to everyone out there who is sharing their SQL knowledge in books, blogs, articles, comments, and so on.