Being a new father, I disagree. It's pretty clear most of the time what's happening with the kids: something happens they don't like, they cry. Something happens they do like, they laugh and smile. Pretty simple. Yes, as infants sometimes they cry without an apparent cause (although it always seemed to turn out to be a bubble of gas!), and certainly toddlers seem to melt at the slightest provocation. But the process is hardly random, in ourselves or in others.

Of course, it gets trickier as you get older, accumulate experiences. Now the same event can carry a unique meaning for every unique individual that experiences it, and that meaning is sometimes very difficult to infer by simple observation.

A tired, cranky kid will pretty clearly be unhappy, but he won't know it's because he's tired and he might even argue with you about it.