(Prefatory note: I've been reading What's Going on in There?: How the Brain and Mind Develop in the First Five Years of Life by Lise Elliot, PH. D.)
We perceive our own emotions in the frontal lobes of the brain, and those areas start operating about six months. Younger babies may feel happy or sad, but, theoretically, they don't know they're happy or sad.
The right side is where we feel bad, and the left side is where we feel good. These areas continue to go through growth spurts throughout childhood, alternating rather than growing simultaneously.
So when your toddler is going through an inexplicably cranky week, you can think to yourself, "Ah, a growth spurt in the right frontal lobe."
Then again, maybe that wouldn't help much.