Formic acid (HCOOH) is a suitable hydrogen storage material because of its high gravimetric and volumetric H₂ capacities. Although H₂ is produced by the thermal decomposition of HCOOH (HCOOH → H₂ + CO₂, dehydrogenation), the production of water and carbon monoxide (HCOOH → H₂O + CO, dehydration) is the major pathway in HCOOH decomposition despite the thermodynamic favorability of the dehydrogenation process over the dehydration process. A large number of experimental and theoretical studies have suggested that both processes are competitive or that the dehydrogenation process has a lower activation energy in HCOOH decomposition. In the present work, we revisit the factors hindering the progress of the dehydrogenation process, using a whole chemical reaction network based on the graph theory. The calculated chemical reaction network shows that the factor controlling the dehydrogenation and dehydration processes is simple and fundamental and can be explained by the oxidation number of carbon and the betweenness centrality. Based on this understanding of the factors hindering the progress of dehydrogenation, the advantage of the dehydration process in HCOOH decomposition is discussed.