Why did Livia leave Tiberius and marry Augustus?

The reason that Livia married Octavian/Augustus is one of history's mysteries. She could not have married him to further herself socially, as she was from a dominant social family, the Claudians, higher socially than the Julians which was the clam of Augustus. In addition, she was also pregnant with Tiberius" child. True love can only be the answer based upon the knowledge that we have at present.