We have studied dissociative H2 adsorption on the H/Si(100) surface by means of pseudopotential density functional theory calculations. This work is directly motivated by the observations of Biederman et al. [Phys. Rev. Lett., 83, 1810 (1999)]: "H2 adsorption is significantly promoted in an interdimer configuration of two adjacent singly occupied dimers." We find that there is no adsorption barrier (in the local site where two quasi-free dangling bonds are available; the adsorption energy is estimated to be 2.6 eV per H2 within the local density approximation. We present maximally localized Wannier functions that clearly illustrate the behavior of dangling orbitals upon the approach of H2. The results suggest that intradimer π-bonding disruption is crucial for the significant promotion of H2 adsorption on Si(100).