People can rationally come to many ethical positions, depending on their value systems.
That's circular. Value systems are not rational. People can rationally come to ethical positions but it has to be based on value systems. The value system it's based on isn't objective, so it isn't logical and rational in the sense of meaning of the word that biomerl is applying.
Why value human life over other life? That's a value. It can't be logically derived except in relation to other values.