Sometimes people ask about Malachi, "Is he always this happy?" To which I usually reply, "Yes. He is a very happy baby."
And it's true. He is. But babies, as a class, are not the happiest of people. When I say, "He is happy all the time," what I mean is, he cries for an average of 2 hours a day. But I'm not being facetious---that really IS an exceptionally happy baby. Isn't it? I suppose I have no comparison except my other two boys. I know Sebby cried MUCH more than that. (6+ hours a day)
In my experience, this is when a happy baby cries: when he's left alone in a room, a while before each meal (as he wants to let you know he is getting hungry), a while before each nap (as he is getting tired), when he's been in the car too long, when someone steps on him, when you don't give him his next bite fast enough, when his diaper is bothering him, when he slams his finger in a drawer, when he wants you to pick him up, when you take away some piece of lint he was trying to eat, when he wants to go up or downstairs (he can't do it himself yet), when you wipe his nose, and when he thinks someone is mad at someone else.
He also occasionally cries for secret baby reasons and no one but him knows why.
A happy baby occasionally continues to cry after he's been fed, changed, hugged, played with, and had a recent nap. But only occasionally.
The difference is that a sad baby cries all of the above times, plus others. And it is for secret inexplicable baby reasons more frequently.