The formation of Li dendrites in lithium metal batteries (LMBs) poses major safety risks and degrades performance through continuous active lithium consumption. Understanding their growth and evolution is therefore critical for improving battery efficiency and long-term stability. Here, we investigate Li dendrite dynamics using operando electrochemical liquid-cell transmission electron microscopy (ec-LC-TEM), enabling direct nanoscale observation during electrochemical cycling. Our results demonstrate that dendrite growth is not governed solely by diffusion-controlled root- or tip-growth mechanisms but evolves toward a stress-assisted local Li deposition regime driven by mechanical interactions between neighboring Li dendrite structures. Li dendrites in the form of whiskers formed via root growth undergo stress-induced deviations, leading to contact, coalescence, and loop formation. These loop-like structures possess two active ends that serve as opposing Li deposition sites, generating growth in opposite directions and producing localized compressive stress. This stress promotes SEI cracking, the formation of new electrochemically active sites, and defect generation, enabling internal Li transport and mass redistribution. Together, these coupled electrochemical–mechanical processes accelerate dendrite growth, drive folding, and govern the transition toward multi-site structural evolution. These findings provide a revised mechanistic framework for Li dendrite growth and offer new opportunities for suppressing dendrites in LMBs.