Weird, I was just thinking about this the other day...
I had not cried in probably ten years, and that included the death of my dog, a broken collarbone, the death of my grandmother, rabies shots and a bunch of other things I'd rather not mention.
Then, I broke down when our first daughter was born. There were some major difficulties and I was totally powerless. A very castrating feeling. I was fine when the doctor delivered the news, I played the role of strong, supportive husband for my basket-case wife. I appeared to be in total control. I walked out of the delivery room to get my parents and my in-laws and the second that I saw my mother's face I was a crying mess. I just broke down and could not control myself.
Shortly after, I had to quit my beloved job so that I could take a better position with another company closer to our hometown. I was very close with my boss and when I told him I was leaving he continued to try to convince me to stay. He was relentless and for some reason the stress fo the situation got me. I started crying in his office. An absolute nightmare situation. Very out of character, but I think the emotional tap was still open from the situation with my daughter.
Anyway, I still keep up with my old boss and he promises me that he doesn't think I am a pussy. He actually respects me for caring that much about the job. I may go back to work for him in the future b/c he has become a serious industry leader.
The only other time I cried was when I watched Friday Night Lights (also around the same time in life). We talked about this on another thread, but for some reason this movie really got me.
There may be something to the OP's theory that we store things inside that need to be let out. There was an emotional change in me after I broke the seal, but it appears to have sealed back up now. It has been a few years since I have cried.