People often see the moral life summed up in the Golden Rule, "Do unto others as you would have them do unto you."
So what if someone were to say "No. I'm me. Other people are other people. I will treat myself they way I want to be treated, and treat them in the way that best suits my happiness. If that means using them for my purposes and leaving them to suffer or die, tough luck for them."
Such a person, I take it, would not be a nice person or an ethical person. Would, however, that person be a person who is not listening to reason? Because that is the consequence of saying that morals are based on reason. If you say that, then you have to say that immorality is irrational.
If you think about "selfish b*****ds, do you think they have failed to be rational, or do you think that their hearts were two sizes too small. Or in asking this question am I bifurcating reason and emotion?