Why do some think viruses are alive?

Caetano-Anolles and Nasir analyzed the protein folds of 5,080 organisms-3,460 viruses and 1,620 cells from other organisms representing every branch of the tree of life. What they found was huge: 442 protein folds were shared between cells and viruses along with 66 folds that were unique to viruses. What this indicates then, is a branching of some kind.

It suggests that viruses were not simply shed genetic material of cells, but shared unique properties with cells (and thus were living) and eventually evolved as separate entities.