Do you want the selfish reasons? Or the selfless ones?

Selfish:

• When you have one of something you love, you usually want more. Like shoes. Only much, much more.
• I want both a girl and a boy - so that I can experience both. Including the baby clothing section of department stores part of having a boy and a girl.
• I think as children grow it's somewhat easier to have more than one, as they can to some extent keep each other busy.
• If I lose one I am left childless. I want to leave progeny. And even if I don't lose a child (that doesn't seem a solid reason) I still want to leave a larger, greater impact on the world.

Selfless:

• I want my child to have a playmate, or a few.
• I want my child to grow up in a world of coexistence, to prep him for future life.
• When life gets serious, especially in adulthood, siblings have the potential to offer a platform of support like no one else.

Those are just a few reasons. Most of the reason is heart not logic. And yeah, this is on a very shallow level.

(I have one son but want another child.)