The paper studies the evolution of contact forces in granular materials as a preferential growth process whereby the incremental change in forces is proportional to their current values. Preferential growth of forces is depicted herein as a positive feedback process that enhances the heterogeneity in granular media over the course of loading. As an initial step, 2D discrete element simulations are performed to verify the hypothesis in the simpler case of hydrostatic loading, with an extension to deviatoric loading thereafter suggested. Not being predicated upon particularities of any mechanical model, the preferential growth mechanism reveals the commonality between the force network and other self-organized complex systems with dynamic interaction networks. The paper concludes with potential mechanical consequences such a dependency between current state and the incremental change entails.