There are a number of possible causes.
Biologically something could be happening, Im not a doctor but as I remember there was a medical reason why no tears would be shed.
Psychologically, you may have put such a tight lock on the idea of crying, no one can see you/hear you do it that your brain just wont LET you cry.
Thats my best guess
They can't touch me while I'm alive, and after I'm dead, who cares?
-John F. Kennedy