Being happy aids in affecting the emotional state of others, for others do not like to be surrounded by unhappy people, but being a selfish animal, you act good towards others, not for altruistic reasons, but because you selfishly derive pleasure from the happiness of those you act upon. When you realize that acting "good", -doing good acts, giving gifts, saying pleasant things, -rewards you with the pleasure of seeing and knowing others' happiness, then acting "good" no longer is conceived as a chore or inconvenient. You realize that the act is the means to the end, pleasure.
You must act to derive pleasure from life, it is not an innate state.
Your decision is made from emotion, not ethics. It is similar to the way animals exist. You don't want the pain of the blow, but this has nothing to do with what is "good", for your decision has made you "bad", by becoming the person who is commiting an immoral act. To prevent personal pain, you have become an inflicter of pain.
You cannot be a "good" person by commiting "bad" actions. Ethics and morality are derivations of actions, not your opinion of yourself. Each person IS a known quantity, they have known rights, of which you would be violating if you choose to be the deliverer of pain. If each person were not equal, then our system of law is based on lies, and then we could all, with impunity, act towards others in any manner we wished. Your concept allows for all manner of corruption and evil to spread, all for the sole purpose of reducing personal discomfort.
To decide to be punched is the ethically correct answer, for you retain the authority of moral law by not becoming a criminal to reduce your personal discomfort. By taking the blow, you have the authority of moral law on your side, which in a society that respects the rule of law, permits you to take actions against the immoral person, with the full force of society on your side.