Can't cry in dreams?

I think last two times I've had a dream where I was crying but no tears actually came out. This time when I woke up I imagined my dream and tears started welling up. Come to think of it, I don't think ever in my life did I have a dream where I cried. Much less actual sad dreams. I don't have issues with crying in real life at all, like I mentioned in my example. Why can't I cry in my dreams? I've never seen this question asked before so that's what led me to post this.