Judaism rejects that Jesus was the Son of God because they do not believe that 1) there is but one God, not a trinity 2) God is and always has been so he couldn't be born or died 3) he did not fulfill the messianic law 4) that Jesus could save anyone (salvation) because they do not believe people are born with sin but sin by committing acts which can be forgiven.
There is the rising belief among Jews that Jesus was not a heretic but a very good man who WAS a Jew. They do not have to believe he was the Christ to know that his teachings were good. They want to clear their names from being the ones who were responsible of turning over one of their own to the Romans.
There are also Christian Jews. These are people of Jewish descent who now believe that Jesus was the Christ, the Son of God.
