I had one of my few best friends die in a car accident 2 weeks ago. I didn't cry either but, I think its because I have the ability to control it. I could have cried if I would have let myself do so.
About 5 years ago, when I was still into heroin, I had a fried overdose and die, right in front of me. I felt guilty because I was the one who sold him the dope. At the funeral, I could tell his family was upset with me, and partially blamed me for killing him. I was kinda scared they might do something to me, to avenge his death.
So, when I entered the funeral room where most of the immediate family was crying, I "let myself cry" as well. I felt it would show that I was truly sorry for what happened, but I also did it for my own protection. I guess I was looking for sympathy.
Point being, although I don't usually cry at funerals, it's because I am able control it. If I choose to cry, I will, and they will be genuine tears, of genuine sadness. Its kinda nice to be able to control that emotion. I can choose when, and where, I want to grieve, and I usually choose to grieve in private. That's my choice and it works fine for me.