Plate tectonics mark the Earth as a geologically unique object in the solar system. Understanding why this global tectonics mode developed in this planet and not in other terrestrial planets, especially Venus, remains one of the grand challenges of Earth sciences. Theoretically, plate tectonics requires weak plate boundaries. In this presentation, I will review and evaluate the various mechanical processes that have been proposed to explain shear zone localization in the ductile middle to lower crust. Most cannot develop spontaneously and must follow localization; others result in a high strength shear zone. However, the development of a microfabric in which mineral phases are effectively separated and form subparallel layers, which is often observed in continental shear zones, holds the best potential for forming weak ductile shear zones. This mechanism is not expected to function in the dry lithosphere of Venus, due to the absence of weak hydrated phases. Indeed, Venus does not display global plate tectonics. However, at rift zones, partial melts could play the role of a weak phase. Accordingly, Venusian rifts are the most earth-like tectonic features of that planet.