Molecular mechanics and dynamics calculations were carried out for asphaltenes and resins from crude oils of different origins. Average structural models were used in order to study the forces that determine the association process of asphaltenes and resins in crude oils. The stabilization energies obtained for asphaltene and resin associates were due mainly to the van der Waals forces between the molecules. Comparatively, the contribution of the hydrogen bonding to the stabilization energy was low. It was also found that few structural changes occur in the molecules due to aggregation. Therefore, the contribution of the conformation changes to the stabilization energy was also very low. According to the results obtained, the stabilization energy of the associates depends on the structural characteristics of the molecules. In particular, the stabilization energy is more favorable for those molecules with lower hydrogen-to-carbon ratio, higher aromaticity, and higher aromatic condensation degree. When different solvents were used to break the associates, this tendency was confirmed. The breaking of the associates occurred for those molecules with higher hydrogen-to-carbon ratio and lower aromaticity. These results can be linked to the experimental finding of a greater tendency to precipitate for those asphaltenes with low hydrogen-to-carbon ratio and high aromaticity.