The question still stands, if someone is "rationally" interested in one's own well-being only, and if someone only cares about other minds to the extent of how they affect your own mind through the natural empathy reflex, then why should they be interested in the well-being of other minds purely for the sake of their well-being? Shouldn't they only be interested in how the suffering of the other minds affects your own mind?

I don't think it's really possible to argue against this idea. If you're only interested in your own well-being, then doing things which do not increase your own well-being will not help you achieve your goal.