Under most circumstances I would not expect it, but try going to a funeral some time.
Check out a man whose mother, father, wife, sister, brother, or best friend has died. He isn't crying - even a few brief tears? There is something wrong with him?
I am a (very) quiet, cool, introspective, deep-thinking, usually very unemotional individual (with a maxim that you never take decisions based upon your emotions, they are a notoriously bad guide).
I went to my father's funeral in 1988, to my mother's in 2000. I tried to fight back the tears both times, but I couldn't do it - a few tears broke the veneer both times. Not like my nephew, who cried throughout my mother's funeral, but enough to be noticed.
And why not? These people mattered - greatly. Not only as parents, but as mentors throughout your childhood, and as great friends when you became an adult. You have lost them for good, forever, it is perfectly natural to give into grief.
We are not made of stone, as tough or as brave or as intelligent as we like to think that we are. We are human, we should not be afraid of our humanity.