Morality may be subjective, but it's everyone's subjective desire to avoid pain, negative emotions, and to avoid preference frustration. Therefore we could operate under the logic that this is the closest thing to objectivity, consensus, the satisfaction of existing preferences, or better yet, avoiding the creation of preferences altogether.
In a just world, one person's happiness would not come at the expense of another person's pain.
Basically what I'm saying is that even if morality is subjective, we should operate under the assumption that it's not. Just because you're not the one feeling bad at any given moment doesn't mean that particular, immediate form of suffering doesn't exist. If physical pain and emotions were collectively shared, the entire world would rope.