In the present paper, I consider the problem of what is the nature of the attractive force observed experimentally in complex plasma crystals. Currently, the most accepted theory predicts that an electrostatic force caused by a wake field is responsible for the attraction. I propose that, in the typical conditions found in the majority of experiments, the wake produces another effect: the distortion of the ion trajectories causes an attractive force via direct momentum transfer (i.e. ion drag force). A dust particle located at the edge of a sheath is immersed in flowing ions. The dust particle is generally negatively charged and causes a focusing of the ion trajectories. If a second particle is located downstream, the effect of the converging ion orbits is to align the second dust particle downstream of the first. This ion flow induced attractive force has two properties that makes it a viable candidate. First, the force is one sided: particles downstream are affected by particles upstream but not vice-versa (exactly as observed in experiments). Second, the new force proposed here is of the same order as the electrostatic wake force and much larger than other forces previously considered (dipole forces).