The study of the origins of altruism is quite fascinating. There is increasing evidence that our cooperative, helpful, and altruistic behaviors have an innate component that we add to by acculturation. Felix Warneken has done some experiments on the origins of altruism and helpfulness and the results are quite remarkable.
In this talk, he describes some of the experiments and shows videos of some of them. The ones with toddlers are fascinating.
Do people who aren’t sociopaths think altruism needs an “explanation” in the first place? I imagine people with empathy necessarily understand the motivation to help, but the framing is always in terms of explaining altruism (implying a lack of it as the baseline) instead of explaining a lack of altruistic behavior (implying the existence of altruism as the baseline). Cooperation is advantageous for social species so that we don’t become tiger food (or, in the case of social predators, so that we DO become lion food) -- possible (though not necessarily the actual ones) selection mechanisms (like kin survival -- my genes might live on even if I never reproduce becasue I e.g. sacrifice myself for the sake of my siblings) are obvious upon even brief reflection.
At any rate, children can be very insightful specifically becasue they lack normative socialization, including many socialized perceptual and cognitive biases, so I recommend asking a young child for help with respect to all manner of problems one might encounter.
Speaking strictly from minimal and non-professional experience (a/k/a my posterior):
I doubt it’s just one factor. Anyone who has witnessed children notes that they want to be involved. Most would agree that infants tend to be narcissistic, not yet knowing enough to be concerned for others. As they grow and learn to be socialized, the narcissism and attention seeking is changed into concern for others.
In the example of the dropped clothespin, the child is gaining attention by becoming involved, while retrieving the thrown clothespin won’t garner that attention. By rewarding positive envolvement, we teach children concern for others, and they accept it because they gain more from it.