Since you have placed this under law and tagged it with criminology, I am assuming that you are asking about why we can agree that some things should be crimes but cannot agree on others. One reason for this is that some actions have clear victims and others are less clear.
We can all agree that one person's actions should not be able to clearly harm another person. That is why we can all agree that murder is wrong. So why can't we agree on whethersomething like abortion should be legal? It is because we cannot agree on whether the fetus is human or not. If it is, then abortion is wrong because another person is being harmed. It it is not, then there is no crime.
One reason, then, is because some actions have clear victims and others do not.