What stems from an adaptation to a feeling is the exact same amount of capacity to feel a completely equal and opposite feeling. According to this hypothesis, a imaginary amount of the capacity to love then gives birth to the equal and opposite amount of say, 'fear' or the first one that comes to mind 'hate'.
Why is it that sometimes, our innate ability to include does not carry on to the same amounts of an equally matched ability to exclude or vice versa? If it were to be in an innate nature to draw lines then why not at the same time one be found that removes them?