In my opinion, utopias are impossible because of human nature. A utopia is a place in which somehow all problems have been done away with. It is a place where everyone can live a life that is pretty much perfect. People have tried to construct utopias on Earth via religious communities or via ideologies like communism. These have failed because (in my opinion) of human nature.
I believe that human beings are just naturally selfish and greedy to some extent. There is no way to construct a society that will keep people from acting in these ways. Because people will inevitably act in these ways, conflicts will arise (for example, when a person or family feels they are being treated unfairly) and utopia will be impossible to achieve.