Interesting article in Slate about why toddlers throw tantrums (nutshell: they have no way to identify their emotions, no experiences to draw on, no way to distinguish needs from wants, and their world is generally extremely confusing).
I think this exchange in the comments sums up the two camps of people who want and don't want kids:
Commenter 1: And this is why I don't have children.
Commenter 2: FYI: the headaches are far outweighed by the good stuff like daily moments of pure joy and awe.
Commenter 1: I don't believe you.