Empathy is an emotional response to the (perceived) emotions of other sentient beings. To empathize with someone is to identify with their emotional state of mind, to feel what they feel. However, when you empathize with someone, it is their actual emotions that you identify with and not necessarily the attitude that they have towards the object of their emotion (example : if you have two brothers whom you love unconditionally but one hates the other as a result of his having hurt him in the past, you can empathize with your brother’s stress without sharing the same attitude toward the object of his anger- your other brother whom you also love).
Hedonism is the idea that happiness and suffering are the only things that sentient beings really value and disvalue, we don’t intuitively realize this because we’re hardwired to associate the objects of our emotions with our emotions. The fact that we have mixed emotions and positive attitudes towards negative experiences and vice versa doesn’t negate this. People would choose not to enter Nozick’s virtual reality machine not because they value the truth but because the idea of living a lie is distressing to them. We don’t always make decisions that effectively serve our interests (pursuing pleasure, avoiding stress) but we do make decisions based on a desire to experience pleasure and an aversion to stress.
If we were to make ethical decisions based on empathy alone, then our only concern would be the feelings of those that we feel empathy for. If their emotional well being was all that mattered to us, then we would judge an action as good or bad only on the basis that it increased happiness/minimized stress or increased stress/deprived someone of happiness. In a world without feelings, nothing would ‘matter’ since nobody could be positively or negatively affected by something. For this reason, happiness/freedom from suffering alone (and not preference, autonomy, knowledge etc.) should be the objective of ethics.