I remember when I was a kid, maybe 1st or 2nd grade, I would force myself not to cry at all because I hated the attention I would get from other kids and teachers. If I hurt myself or something, I would focus so hard on not crying, that eventually I just stopped all together.
It used to kind of creep me out when I got older, because when I was about 10 I was at my grandmother's funeral, and I was sitting there thinking that something was seriously wrong with me because I was one of the only people not crying.
I haven't properly cried in about 5 years, and that was when my cat died. I cried for about a week straight. I guess it all just built up and finally released itself.