Speaking as someone who was raised to be fanatically independent (the take home lesson of my childhood was "If you don't take care of yourself, no one will" ) I actually concluded the opposite: codependence must be learned, because you never see small children confuse their best interests with someone else's. Basic human nature is to look to your own needs first. The problem comes in when children are rewarded for being caretakers, or taught that self care equals selfishness, as posters have mentioned.
As an adult, now I look around and try to keep in mind that no matter what people are like, no matter what their maddening behavior patterns are, there's a reason for it.