Because sometimes our reasoning fails?
Are you seeking specific (preferably common) ways in which "mathematical reasoning fails"? If so my first idea for something like this would be failure of short term memory.
It's less, I feel, about reasoning than it is about attention. Working memory, that is.
I'm mostly asking this open question to those among us who are well-versed in developmental psychology (I'm mostly thinking of children) . Although, failing the actual scientific research on the topic, I guess some testable hypotheses would be great too.