Lubricated surfaces have shown promise in numerous applications where impinging foreign droplets must be removed easily; however, before they can be widely adopted, the problem of lubricant depletion, which eventually leads to decreased performance, must be solved. Despite recent progress, a quantitative mechanistic explanation for lubricant depletion is still lacking. Here, we first explain the shape of a droplet on a lubricated surface by balancing the Laplace pressures across interfaces. We then show that the lubricant film thicknesses beneath, behind, and wrapping around a moving droplet change dynamically with the droplet's speed - analogous to the classical Landau-Levich-Derjaguin problem. The interconnected lubricant dynamics results in the growth of the wetting ridge around the droplet, which is the dominant source of lubricant depletion. We then develop an analytic expression for the maximum amount of lubricant that can be depleted by a single droplet. Counterintuitively, faster-moving droplets subjected to higher driving forces deplete less lubricant than their slower-moving counterparts. The insights developed in this work will inform future work and the design of longer-lasting lubricated surfaces.