What is lacking in a virus which makes it dependent on a living cell to multiply ?

The following are the reasons why virus need host cells for replication:
1) Despite the fact that viruses carry their own genome in the form of DNA or RNA molecules, they lack the necessary ribosomal RNAs and ribosomal proteins required for the process of replication. This is why viruses take up the cells host protein building mechanisms to form their own viral copies.
2) Due to the viruses' simple nature, they require energy from their host cells to drive the process of their replication. This is because they lack the ability to manufacture and use up the energy.