The fullness of human life comes from the realization of humanity in its fullest. The metaphysical decision that humanity is full at any one point is a difficult and incomplete base to build a morality off of. Just one simple example, if a huamn fetus missed some chromosones or otherwise didn't meet some criteria of "sperm joining egg", would it be considered full or not? Recognizing helplessness is only part of empathy. A fetus of another animal is just as helpless.
I can explain it to you but I can't understand it for you. It has nothing to do with helplessness. It is about humanness. If that has to be explained, it cannot be understood.