Ethics (or morality) is concerned about what people should be doing, what is good and what is bad. While what is morally good or bad is ultimately subjective, because every person has their own set of behaviors they see as acceptable, a significant portion of that is based on things which are objectively (or commonly to life) good or bad. For example, suffering is objectively bad, because life always avoids it. It is undesirable by its nature and definition, so everybody thinks it is morally good to minimize their own suffering. However, that does not mean it is also objectively bad to cause suffering to others.
Before we do something that involves another person, we often ask for their consent. Asking for consent is about finding out, whether the person asked has the same moral values as you. If there was a copy of you, you would have little need to ask your copy for consent, because you both have the same idea of what should be done. Therefore, even though some people see consent as necessary, it is not objectively important and in practice it is sometimes broken for the person’s benefit. For example, helping someone, who is unconscious and injured due to an accident – "accident" implies that the person did not harm themselves intentionally and so it is reasonable to assume that they would consent to the help, if they could.
While it is not objectively true that causing suffering to others is morally bad, from the laws of human societies and general behavior we can conclude that most people would agree causing suffering to others is in many cases morally bad behavior. It makes sense to not want people causing suffering to others, because by doing that you are making clear that you find it acceptable and so other people may do the same to you, which you don’t want. This is related to a sense of fairness, which is another moral concept common to life.
Some people may say that a person deserves to suffer as a punishment for their actions, but because our actions are determined by our past, the actions are not the person’s fault in a moral sense, so nobody can really deserve to suffer. It does not make sense to punish people with the sole purpose to make them suffer.