I have a theory but I will just present the question and let people think about it.
People won't admit it - but the reality is that a lot of parents love their grandchildren more than they love their own children. It shouldn't be this way because halachically and morally one's first obligation is to his or her children but this is the reality. 99.9999 percent of people live based on the dictates of their natural emotions [the remaining fraction makes sure that their intellect and moral code guide their emotions] and if they "feel" more love towards their grandchildren then they are going to be the main recipients of their love and not their children. But why would many people feel this way?
