Why can’t we predict when a volcano will erupt?

A paper that I published in Nature Geoscience demonstrated that volcanic eruptions in the hazardous Campi Flegrei system (Naples, Italy) may be triggered by the expansion of gasses, which saturate in the magma shortly before eruption. This was discussed in an article by Prof. David Pyle in The Conversation. The paper was also covered by a number of media outlets, including the BBC,  IFL Science and the Mail Online.