How come everyone is missing the obvious answer? The human ancestor that first developed attachment to babies may be an ancestor we share with rabbits.

(Edit, Also: Human babies may have evolved to be uglier for other reasons -less hair, bigger heads- and those features may have been selected for more than cuteness.)

Edit 2: Metaphorically, our cuteness program is like running Netscape Navigator 1.0 or something. It sort of does the trick but isn't exactly adapted for modern uses