Theory and experiment suggest that the flocculation of non-Brownian, flexible fibers in flowing viscous media arises from the mechanical entanglement of fibers. We probe the role of frictional and attractive interfiber forces on fiber entanglement using a recently developed particle level simulation technique. With frictional and repulsive interparticle forces only, simulations capture flocculation in 0.125% by volume suspensions of fibers with properties similar to those of softwood pulp fibers. Here, model fibers elastically interlock-flocculation tendency and hoc coherency are directly linked to elastic energy storage in fibers. By applying only repulsive and attractive interparticle forces of various strengths, sheared suspensions attain similar degrees of aggregation, and similar flee strengths, as in suspensions with interfiber friction. However, in the absence of interfiber friction, model fibers do not elastically interlock. We find friction to be an essential element in elastic entanglement of fibers. Simulations suggest that effective formation aids in papermaking should reduce interfiber friction under normal force loadings of 1-10 mu N.