I completely agree....I cry out of anger, I'm very familiar with this. Many things can cause a person to cry...anger, guilt, stress, sadness. If he was doing it in his sleep....it might have been none of the above and merely his bodys' way of releasing stress....that happens too.
At some point today I will ask him if he was crying last night. If he truly was I'll ask him why and see what the answer is, if he says no....then I'm going to chalk it up to he was sleeping.
But like I said....not going to analyze the why's on this one.