Both of the above (=Bentham's classical utilitarianism)

I mean this.

In any case, what answer do you expect?

I do not expect any specific answer.

What would constitute a valid reason?

For me personally, probably nothing, since, apparently, I neither really care about people (I guess I overintellectuallized my empathy), nor about pleasure and suffering. The question, however, was asked mostly to better understand other people.

What are the assumptions from which you want to derive this?

I don't know any.