At some point, someone in a position of authority (a teacher, parent, family friend) either implied or said outright that some people are "Math People" and some people aren't. OK, by that logic, would you ever accept someone refusing to read an assignment because they aren't "Reading People?" Of course not! If we don't accept illiteracy as a valid response, then why should we accept an inability to do basic calculations?
I came across this article yesterday: Key Differences
While there are some assertions that I don't agree with in the article (for instance, I'm not convinced that year-round schooling is the panacea that some think), the essential gist is completely valid: mathematics can be learned, regardless of innate ability. The key is how you approach the subject.
Answer this question: If you are having trouble learning a new skill, what does that mean?
a. You are stupid and cannot learn it.
b. You need to try harder.
The problem we face as teachers is that the first answer, the path of least resistance, is treated as truth and accepted into the personal myth of each student. This non-critical thinking gets passed on to other subjects- I can't draw, I can't design, I'm not good with computers...
Of course you aren't right now...but you could be. Try harder.
"Anything worth doing is hard." - Wil Wheaton