N-body simulations show that dark matter haloes develop from the inside out in phases of smooth accretion between major mergers. In these conditions it can be shown that the density profile of ellipsoidal anisotropic rotating haloes is completely determined by the current values of their extensive properties and their corresponding accretion rates. This allows us to build a simple physical model for the evolution of the NFW scale radius. Moreover, since we can arbitrarily assume that a halo has grown by pure accretion, the inside-out growth condition automatically leads to the halo density profile. We find that the typical accretion rate predicted by the extended Press-Schechter formalism in any given cosmology leads to a typical density profile of the NFW form for the correct mass-concentration relation.