Pain
Pain matters as a way to judge moral consideration because we value pain in ourselves and in people that experience pain like us. It's an evolutionary mechanism that evolved in humans and a number of other organisms to serve as a signal to change our behavior to avoid harm.
A lot of value comes from a human life outside of our pain as a signal. We value relationships, love, the experiences and suffering that we go through that make up human consciousness, a concept that we don't even have the ability to understand entirely.
We're all aware that this can exist only due to the complexity of the human nervous system, which comes in stages as the fetus develops in the womb. The question I ask, why do we assume that something without the ability to feel pain, that has only started developing its nervous tissue, has moral value of any kind?
A person or animal's rights don't inherently come from their feeling of pain, but that their ability to feel pain serves as an evolutionary mechanism that incentivizes other people to protect them in order to avoid pain and suffering for themselves.
In the case of someone who can't feel pain, we would say that moral value comes from something else, such as their cognitive function or how strongly we identify with them. The latter, of course, would be subjective and not objective.
