>>804109Seemingly, you do not need a reason for that, as moral valuations are firstly felt in the intuitional sense before it is justified and reedemed in a self-discourse with the absolute, the personal god. (Call it science, religious law, etc. what is the common logic)
Perhaps, when we do think about causality, the material conditions, aka the nature, may have been creating those moral-judgements. As we can simply propose: the mind cannot take hold of an object that is not can't be in a relation, just as how we can't foresee a new colour we never saw before. Nature(both our body & its outside) in that sense restricts or changes what we can feel, and what we can interact with.
Equally, when you do ask
"Why is it not okay to kill animals? (Why killing itself may considered a sin?)" , we simply stress the same moral question which we have a hard time explaining. What would make the x judgement superior than the y judgement for our moral question?
Regardless, we still have our judgements, the cultural view / volkgeist adopted by the society(-ies) are of course open to discourse (and perpetual changes), but regardless of that, we are also subjected to the society, and nature in general. Lovers do kiss, but why do they do that, as merely in the act of kissing there seemingly lies nothing, yet for some it is the rite of uniting between two souls. What is the truth of it?